This is a tricky one...If you were BLINDSIDED and was attacked and you tried to defend yourself then I'd think the school would understand. If this is not the case and it was premeditated or not on both parties, I'm sure the school will investigate and take proper measures to fit the punishment.
If you are truly defending yourself, ad your mother is prepared to sue the school and makes it know to them, you are good. But considering the fact that most schools are stupid and psychotic, if your mom is not helping you..... You are screwed...
Typically, all members of the fight are suspended, regardless of the situation. If you hit back, you get suspended, period. It depends on your parents what their reaction would be. If someone is threatening you or bullying you, talk to your parents, or report it to the principal immediately.
Well, chances are that the staff wouldn't look on it favourably, but hey, they're not the ones that were threatened. You'll probably be punished, but it's worth it to protect yourself. That's my opinion, anyway.
Getting into a fight and defending yourself are two different things. The first lesson you learn in any martial art is to walk away from confrontation. Defending yourself can be easily done without throwing punches if you have received proper training. If you fight at school, be prepared for suspension.
Don't get into the fight until you have to. Don't start it. If you need to defend yourself than do it. It's for your own good. But you will probably get suspended. Just make sure you kick butt so it's worth getting suspended