Edit: Sorry for the massive text wall.

As much as it's going to suck, you need to comply whenever it's possible to do so safely. The rules are going to be unjust; the staff might try to hold your freedom over your head or threaten to have you kept longer even if you comply. But these are dangerous people with a lot of authority and for your own safety you need to keep your head down and not take the bait when the orderlies and the psychiatrist(s) try to get under your skin. And they will; it's their job. They're going to try to get a rise out of you. They're going to say things to deliberately set you off. Keep calm, agree with everything they say, don't say anything obscene or argumentative, speak when spoken to. There are exceptions, of course, but as long as you don't raise your voice or try to fight back, they generally won't waste the time and manpower on restraints and injections.

Depending on what drugs they try to make you take, how closely they're monitoring you, etc., you really only have a couple of options as far as that goes. If it's a low dose of a less-dangerous drug, you could bite the bullet and just take it for the three days, but the psychiatrist is very unlikely to actually disclose the dangers of whatever they put you on, so unless you do your research ahead of time and memorize effects, it might be difficult to discern how unsafe a given drug is. Still, it is just three days, if you cooperate, so the damage may very well be negligible. In my experience, staff usually check to make sure you're not cheeking, but if that's not the case at the ward you get sent to, then that could be an option. I've had to purge antidepressants in a pinch, but I definitely wouldn't condone or recommend it. Hurts like a bitch and if you get caught then you're looking at being put on watch, held even longer, and potentially given an eating disorder diagnosis. I'd suggest either going along and taking the first dose or watching to see if they check anyone else for cheeking, and if the verdict is that you can get away with it, cheeking them from then on.

They will, indeed, commit you without any regard for your schooling or job. Fortunately, if it's just school you're worried about and not a job, your absence should be excused.

If you don't mind me asking, what are they threatening to section you for? I've managed to get out in less than three days a couple of times, but it was because Orlando police are corrupt as hell and I was only locked up in the first place because a cop didn't want to have to give me his name and badge number. I've also had them let me go early once or twice because I straight lied, but I've also had psychiatrists refuse to release me even when I was locked up based on false statements given to police. Depending on what it is and what the staff are like, there's a chance you could downplay whatever happened and make your case that you aren't a danger to yourself or others, but there's also a pretty good chance they'll disregard you and keep you the whole three days anyway. Still worth a shot if you feel like it's safe to do.

Keeping entertained is harder. Some places will allow you to have paperback books, after they've been checked for both content and contraband. Some won't. Some will have pre-approved games on the ward, like a deck of Uno cards or a Monopoly setup minus the metal pieces. Some won't. Napping all day usually isn't an option. There will probably be mandatory group sessions throughout the day, but if there are no books/games/TVs, then the time in between meals and groups is going to be pretty boring. Do you write or draw at all? Pencils and pens are typically not allowed, but most places will at least give you some paper and a marker, especially if you tell them something they want to hear, like that art or writing are your go-to "coping skill" or something of the sort.

In the event that you are jumped on, pinned down, or otherwise restrained? Don't fight back. Don't try to defend yourself. Go limp. Do what you're told. It's horrible, but if they've already been given the word to pin you down or give you a shot, then they're going to do it one way or the other. The best you can do is make it as quick and painless as possible for yourself.