What can a teacher/principle do to you if you leave the classroom without permission? Can they do anything?

See my answer on your other similar question.

Now I'm even more sure that you're just some child that thinks that they are an adult. Legally or otherwise.

Here's one definition of truancy:  The absence of a child from school without permission.

Bottom line, if you want to remain a student at that school, you will have to follow the rules or they will eventually expel you.

So they  can do various things to motivate you to comply with their rules or they might simply send you a letter saying "Goodbye, Have a good life."

There are rules in schools for a reason  (especially if you are a minor) .. If your school happens to have a permission to leave the room rule, and you refuse to abide by it .. Then there are consequences for being beligerent and non compliant.  As long as your school is responsible for you and your safety they have every right to expect you to follow the rules .. If you refuse to do so, then the school may have no recourse but to take action which could potentially result in suspending you from coming to school property so the responsibility for you and safety remains in your parents charge.

If you have authority issues or you want to be treated 'special' and not have to abide by rules that everyone else has to ... ?  Go ahead. Just be careful what you wish for .. Cause you just might get it .. (and it won't be in the form that YOU are thinking).

I'm thinking you might have a lot to learn how to integrate into the real world .. But you will learn one way or another .. Even if you are a slow learner .. You will learn. Prisons are full of idiots who think on they don't have to follow the rules.

Your teacher and principle can ask your parents to meet due to your mistake. You are not allowed to leave the classroom without the permission of a teacher.