Go back to your high school with your parents and a medical explanation by your general practitioner or psychologist. If you can afford it take a lawyer with you who does cases related to schools. If they want, the school administrators could admit you back into school, but you will have to fight against a unnecessary expulsion, and may have to face a hearing.

However, you need to behave yourself and you never really revealed how belligerent you were or how you behaved in school when you were expelled. Irrespective of that, you have a right to an equal education and that is your defense. By the way, they may suggest tutoring at home with a teacher. They may also suggest special behavioral classes or another form of an alternative school. Your lawyer will help you with that. Good luck.