WARNING: If you get caught doing this then it's not my fault, I merely tell the information and don't endorse doing this at all. Proceed with caution and do not try this at home/school/anywhere.
P.S. If you have followed me for my art content just that stop reading here, as this will probably not interest you.
How to break into a school:
1: This first step can be done in many ways (beware that A is pure luck)
A: Get into trouble and get sent to the principals office, when you are in the office getting grilled about how you shouldn't do blah blah blah, try to take in as much of the room as possible (beware of cameras) now cross your fingers and hope that the principal or whoever is dealing with you (must be a high position in the school only they have master keys) gets interrupted and is sent out of the room to do something. Hopefully, by now you have located where his keys are and can quickly use an impression mold thing to get the shape of the key. look up "Duplicating a Key from a Photo" on YouTube for another way to do this part. Once you have created the copy cat keys you can continue on to the next few steps.
NOTE: You could get a friend to do something that forces the principal to leave his office, but that won't always work so only try that if the friend knows they might get into trouble and get nothing out of this, if they are a bit uneasy about it but still agree, promise them an award for doing it whether the plan works or not.
B: (This next step will only work if your teacher has a high role in the school, because as said before they are the only ones who get master keys) Sit close to the teachers desk and when the teacher goes out of the room to do some task (which will happen eventually) grab the keys and refer back to the end of A. Beware that this method can fail if someone rats you out, keep mouths shut about it. Also, remember about cameras.
C: This method is probably the easiest and least likely to get you in trouble. Sometimes if you are a good student you will be trusted to use your teacher's keys to run an errand for them. Whether it be fetch some documents from the teacher's office or go get your bag from a locked class. Refer to A and continue on once you have the copy cat keys.
2: The hardest part is basically done. Now what you need to do is make sure you know the lay out of the entire school, where cameras are, know how long the teachers stay at the school after the home bell and also when and where the janitor does what janitors do. Now I'll explain what to wear. Don't have any skin whatsoever showing, hunch down a little so anyone who looks over camera footage doesn't have a good estimate on your height. Where good shoes, if all else fails and you are seen immediately leave the school campus. Don't head straight to your house as you might be followed, go a longer route and try to take as many turns as possible to throw off anyone that could follow you. A good pair of gloves is needed so you don't leave any finger prints. Speaking of finger prints once you have done whatever you want and have finally come home immediately dispose of all the clothes including shoes. Wear a full face mask (nothing too flashy, any witnesses will probably remember the mask you wear, keep it simple and generic). Have some kind of special eye wear that shields your eyes but doesn't blur your vision (eyes are unique and can easily get you caught if a witness remembers how they look). If you want to carry a knife for use in sticky situations go ahead, but never EVER use it as a weapon. The best way to describe this is: Robbing a bank is very serious and dangerous business but as soon as someone gets attacked or killed in a robbery, police take it 10 times more seriously. And if it isn't obvious enough, don't use a gun they are loud and will attract a lot of attention.
3: Have an escape route planned. Every second count's you can't be dis organized or you will be caught.
That's about it, if you live in America I shed a single tear for you if you get caught. If you have any questions, by all means, ask in the comments.