In a story that isn't set in our normal here-and-now, be it fantasy, science fiction, historic fiction, or something else, you need to establish what's normal for your setting, and what isn't. As an example: aliens land in the local spaceport - is it an "inciting incident", or are they just regular traders? Or is landing of aliens in general commonplace, but those particular aliens are a surprise?

For your reader to understand what's "out of the ordinary", they need to understand what is "ordinary".

In the absence of evidence to the contrary, readers will assume "our normal world". @MatthewBrown mentions Hamlet as an example: indeed, we know what court is, and what court politics are. Thus, the opening scene quickly establishes "we are at court", and moves on with the story. Ghosts, we are told, are not normal. But the inciting incident is not "the ghost appearing", but "the ghost talking to Hamlet", which doesn't happen until we've been introduced to Hamlet's "normal" at Claudius's court.

Game of Thrones starts in a similar manner: in a quick prologue it is established both that the Wall is normal, and that the Others are not. (Note that in this case, this is not the inciting incident - the inciting incident is King Robert coming to Winterfell.)

The more your setting differs from our "normal", the more you would have to establish right from the start. But then, the very fact that it is all different makes the reading interesting (providing your writing isn't boring). You thus have some leeway for a longer "beginning". As an example, nothing much happens in the first chapter of The Lord of the Rings - "The Long-Expected Party". But there are hobbits, and there's Gandalf, and there are hints of other things, so that makes it interesting enough.

All the same, starting with "normal" doesn't necessarily mean starting with boring getting-up, brushing-teeth. As an example, Jim Butcher's sixth novel of the Dresden Files series, called Blood Rites starts

The building was on fire, and it wasn't my fault.

My boots slipped and slid on the tile floor as I sprinted around a corner and toward the exit doors to the abandoned school building on the southwest edge of Chicagoland. Distant streetlights provided the only light in the dusty hall, and left huge swaths of blackness crouching in the old classroom doors.

[...]

I checked behind me.

The guardian demons looked like demented purple chimpanzees, except for the raven-black wings sprouting from their shoulders. There were three of them that had escaped my carefully crafted paralysis spell, and they were hot on my tail, bounding down the halls in long leaps assisted by their black feathered wings.

In this case, the fast-paced chase scene establishes the "normal": the location is urban-fantasy Chicago, the protagonist can cast spells, and monkey-demons throwing flaming poop are not a surprise for him.

For your setting, what is normal, and what isn't? Your character is going to school - that, supposedly, is normal? Or is it that for some reason your character wasn't expecting to be going to school?

Buffy starts with Buffy going to school (actually it starts with establishing the existing of vampires preying on students, but then it starts with Buffy going to school). Then there's a vampire-eaten corpse in a locker. There's a clear delineation: there's "normal", there's "weird for most of the world, normal for Buffy", and there's "so weird, it's weird even for Buffy".

Yes, please start in the MC's Normal World.

The point of beginning in The Normal World is directly related to the inciting incident: Namely, the inciting incident has the potential to change the character's life. For good or evil. Whether they like it or not. It may change it immediately, or it may grow to to change it. In all these cases, the inciting incident is going to take the MC out of their Normal World to deal with it, and learn from it, be abused by it, and eventually solve it.

After that, they will return to some Normal: Sometimes their previous Normal world (e.g. the villain is no longer a threat) or more often to a New Normal World (It is new because they have changed, sometimes become a better person, taken responsibility, or become more aware of the World or more Adult and decided their old Normal is too small, or they have a higher purpose, etc).

In order to make the Inciting Incident matter to the reader, the reader should already know the MC, sympathize with the MC, and understand what the MC is facing when their Normal World is disrupted. If they don't know the MC,* then by definition the MC is a stranger and you have something happening to a stranger they don't know and don't care about. They expect heroes and villains, they don't know which they are looking at. It doesn't seem like a person to them. They don't have any sympathy, yet, they don't know if they like her.

Further, the second reason for starting in the normal world is that in the first pages of a book, readers expect and forgive a little world building, that authors will be more careful to include setting details and the rules of the world (like whether magic works, like whether we are on Earth, like what time-period we are living in, from cave-dwellers to far future, like cultural details, even whether we have mobile phones and the Internet or not).

Those details need to be incorporated to orient the reader's expectations, but they will also get in the way of dramatic scenes, reducing the impact of them. Even if your setting is entirely modern present-day Earth, your MC has a local culture, attitudes, and permissiveness that readers need to know about. A reader living San Francisco California has a much different "Normal World" and expectations than a reader living in Laredo Texas, or one living in New York City.

They are all flexible, they read for entertainment because they LIKE other worlds, but you need to SET their expectations for YOUR story.

Typically, instead of the Inciting Incident, I open the story with the MC in their normal world but with a normal world PROBLEM, something minor that isn't life-threatening or life-changing, just some issue to deal with like we all encounter in normal life. Something breaks or doesn't work, or we run out of something, whatever.

As I've said in other answers: We wake up late because there was a power failure while we slept, and our alarm failed to go off. Or there's no hot water to take a shower. Or we usually have a bowl of cereal for breakfast, but when we pour milk on it, the milk glops out: It has gone sour. Or we have pop-tarts, but the toaster is broken. Our shoe breaks, or shoelace breaks, we have to improvise. Our car won't start; the battery isn't dead, and that is the full extent of our knowledge about trouble-shooting cars.

A small low-stakes problem gives the reader something interesting to follow, to see how the MC behaves in her normal world, how she thinks, if she is creative, etc. We are able to introduce the setting, because the stakes aren't high, and the danger is not life-threatening; it doesn't feel like we are pressing pause on a life-and-death action scene to explain how a light-saber works, or that Luke thinks Darth Vader killed his father.

In most novels the Normal World is the first half of the first Act, which amounts to 10% to 15% of the story. It can be shorter, but without it, stories generally flop, because the reader doesn't know the character, doesn't get what's wrong, doesn't see the Inciting Incident as a change from Normal, and when you try to justify the MC's feelings and actions you inevitably stall the story with backstory.

Readers aren't stupid, they are flexible and ready for anything when the book opens, they are expecting you to set the stage and introduce an MC (or more than one) and supporting characters and culture. If you don't give it to them, they will not find your story Exciting because you jumped right into the action, the will find it confusing and flat, because they don't care about the characters involved.

In The Hunger Games, Katniss is introduced in her weird Normal World, shown to be an expert huntress, shown to love her sister, be afraid of the drones, etc. The Inciting Incident (foreshadowed by nightmares of her sister) is her sister being chosen for the Games; the only way to save her is for Katniss to volunteer in her place. The equivalent of a mother sacrificing herself to save her child.

In Die Hard, John McClane's Normal World is visiting his estranged wife and daughter and trying ineptly to repair his marriage. The Inciting Incident is his family being held hostage, and it is 100% action from there: But we did not OPEN with that, and have to explain "By the Way, his wife and children happen to be hostages too! And he loves them so much, he will run barefoot over broken glass to save them!"

The Normal World introduces the MC, introduces the World and Setting of the story as they know it, and introduces the stakes for the MC.

Don't skip it. Don't think you are starting at the exciting part. That is not what In Media Res means, it means don't start the story with a lot of factoids and exposition about the history of the world so far; it means start in the middle of things, start with characters doing things, start with some sort of activity, but it doesn't have to be a battle, or the inciting incident. Start with the MC(s) doing things that show us the setting and other supporting characters and the stakes that will be threatened. The Normal World.

You do need to establish your character's "normal" -- but you almost certainly don't need to do it in a lump at the beginning of the book.

The Hobbit doesn't start with a long chapter about daily life in The Shire; it starts with Gandalf showing up after only a few paragraphs describing the physical setting. The deep description of The Shire is doled out in dribs and drabs leading up to the visit of the dwarves, and even moreso put off until the first few chapters of The Fellowhip of the Ring -- roughly seventy years after the events of The Hobbit.

Nine Princes in Amber doesn't start off with a description of Corwin's "normal" -- either the one he'd established on Earth, or before then, back home in Amber; it starts with Corwin waking up in a hospital bed, with amnesia. It takes five books to get a reasonably complete description of Amber, and of course no deep description was needed of ca. 1975 Earth, because readers lived there.

In short, many very good books fold the "establishing shots" into the story, because this draws the reader in much more effectively than starting with "I got up and made coffee" (although in fact the first book of the second Amber series, Trumps of Doom, did start with that, by way of explanation about what was special about that particular day).

A critical point that other answers skip over is: Where you start writing and where the finished story begins aren't all that related.

Pick a point within your storyline, and being testing the story. Read it to yourself. Have others read it. Have others read it to you if you can.

Then keep asking if the story works. And it really doesn't matter if this is done with 'final draft' quality writing, or just napkin scribbles of ideas for a story outline. But I will say that the earlier you start getting into this kind of mindset, the easier it is to spot and correct problems.

While not the easiest 'answer' to a question, the honest truth is that you answer this question by testing against your story, and you can't effectively answer it by polling random strangers for highly out of context opinions. Answers here can give you a lot to think about and consider, but ultimately it will come down to what works for your story, and its style, feel, and flow.

• Does the reader know enough about the characters and the world at any given time? If not, look at how to address that.


• Does the reader get bored with info dump and excessively mundane content? A chapter about a Main Character sitting on the toilet dreading the upcoming work week [Possibly followed by a few more chapters about said work week] might work for one story, but utterly fail for another. Advise that is 'true in general' may not be true in a specific case.


• Does the book as a whole actually work at all? The greatest beginning to a story won't help it if the thing as a whole just falls apart into a useless mess.

Your readers should have an understanding of normal for your world, but how you introduce that will depend on your story, pacing, and characters.

Starting with some normal is typically a safe point to begin, but it doesn't need to be dragged out to excess, however it is not a strict rule written in stone. [But still be prepared for things to 'not work'.]

• Characters can reflect on normal or otherwise reference normal through dialog.




• Flashbacks to normal can be used to "fill a reader in" mid way through the story as needed. [But this can be difficult to do well without descending into a tangled mess.]

In short, you can 'establish normal' in far more ways than "starting off the book with how dull and boring things are before suddenly getting exciting" if it works for your story.

The key takeaway is to be wary of judging too much of a book before it is written, and be careful with writing to excess before you start evaluating it. While that sounds like a conflicting message, it is really mostly about aiming for a balance.

And remember that it is often easier to decide a beginning 'needs more' than it is to acknowledge that the opening needs aggressive trimming.

• Frame up the beginning of your story, but don't finish it till you're finishing the whole story.

[It is far easier to let go of bad ideas if you haven't invested too much time in polishing them before you're in a position to judge how bad they really are...]

I would strongly advise against adding chapters into the beginning. Of course, it has to be said, I am not you and you are the artist. However, let me explain why I would not do that.

It is often said that a story should start in medias res (in the middle of the action). By starting in the action, you begin where the story is most interesting. This can be an amazing way to hook a reader and keep them interested. If backstory is later important, you can drip it in as and when. Take Hamlet, for example, we start the play with his father already murdered and his mother having remarried (to the murderer).

As you ask about the advantages and disadvantages of starting before the inciting incident I will try to list them:

Advantages

• You can dump in a lot of backstories


• You can show the reader what the protagonist has to lose


• There is an opportunity to give the inciting incident more emotional weight


• You get to fill the reader in on stuff they "need to know"


• You can show a lot more of the "before".

The reason Harry Potter starts in the mundane is that this is where we live. We then get to follow Harry from the normal into the fantastical. He takes us with him for that adventure. We need to find out about the Wizarding world and can do that with Harry. We understand how motivated Harry is to leave behind his crummy life - something that is key to his journey in the first book.

Disadvantages

• Very easy to bore the socks off your readers


• You have to find a reason to make all the "before" interesting


• There is a risk of unexciting early chapters full of info-dumps


• The reader has to wade through all that to get to the "exciting bit".


• Your story can become more about "normality" than the adventure.


• If you screw it up, there is a chance no one will read long enough to find out why the middle is so good.

In Hamlet, we already know what murder and politics are like. This is a story of someone contemplating revenge, so we start with them there. We pick up the details of the murder as Hamlet sets about proving to himself that he should take revenge. Had we started a year earlier, there would have been a lot of stuff happening none of it relevant to Hamlet's revelation at the start of the play.

Many books do not start in the normal world. There are even books that start by showing the end and then narrating how that end came about.

What you need, though, is to understand what effect beginning at a certain point in the narrative has on your readers and consciously decide on that beginning.

1. 
   

   Most action or suspense focussed stories begin in medias res.

   
   
   

   This appears to be what you plan to do. These stories usually begin in a world and setting that is either familiar to the reader (i.e. our own) or conventional (e.g. a typical Science Fiction battlefield in outer space). While you may not notice, these setting are in fact introduced, but only minimally, by using certain keywords or brief descriptions embedded in the action (e.g. "John pulled out his laser gun and fired at the space ship visible outside the breach in the hull.").

   
   



2. Most epic tales and every Bildungsroman – that is, every narrative that focusses on he psychological and moral growth of the protagonist – begins with a slower setup against which the changes can be contrasted. The description of the normal world serves as a 'baseline', so to speak, against which the changes become visible and meaningful.




3. Stories whose main plotline is the solving of a riddle, such as certain crime fiction, sometimes begins with an event before the protagonist becomes involved in the story (e.g. we see the murder taking place in a prologue) or with the final outcome of events (and the path to the end becomes the surprise).

There are many other possible options and effects. Think about what kind of story you have, and what drives its plot.

Every recipe has a reason.

The Heroe's Journey and the starting in the ordinary world has a very simple reason: It works.

Notice how it is especially used in fiction and SciFi, because the gombdoggle exchange blaster showacks with the Truklacks in a huge Kugla ship at the edge of the Irofan space is completely incomprehensible unless you've introduced these elements first.

The ordinary world helps the reader to orient himself, to understand who the hero and a few other characters are, where the story is set, what the environment and the rules of your world are.

It also makes the reader relate to the protagonist and to understand why the extraordinary that your story is about affects him so deeply. You need to somehow take the reader on the journey, and easing them into it is the most reliable way to do so.

I'm going to suggest folding in "normal" over the course of the story, not doing a big info dump. I would also warn against, when incorporating it, doing something like "When little Susie was younger, bigger kids seemed nicer, but not now!", since it doesn't feel as organic.

If you want a good example, Log Horizon is a story where people are transported to a fantasy world based on an MMO, perhaps the biggest possible "inciting incident," since it is the biggest change possible (their bodies, world, and economy have suddenly changed). What sets it apart from it's contemporaries is that it starts off with an action scene in the middle of the first arc that hints at the inciting incident, then reintroduces us to the new setting with some off-hand remarks about how this is "slightly different" than what they were expecting since it was originally a game world to them (i.e., thinking of skills instead of clicking on them) with only one mention of an incident, then using real-world holidays and brief mentions of "the incident" as the only real mention of the normal world until that outside world becomes relevant to the story, when we get a few discussions and a dream sequence.

A New Hope has a much shorter establishment, also not in the beginning of the story, that establishes Luke's normal life as a farm boy after the main conflict has already been introduced.

Many detective novels start with the main character hanging in their office, then getting a call or a letter or a visitor that kickstarts the mystery. Since you have a more grounded story, a good way to establish the MC's normal in an organic and chronologic way (if you're into that) is to start off the morning of or at most the day before the first day of school. You can use this time to lay the groundwork of who the character is, what they want, and foreshadow the eventual conflict.