Suing the school is probably pointless, in the sense of actually winning the case. It would be far-fetched, and more of a negotiating ploy than anything else. But if they are already expelling you, then you have nothing to lose. Maybe they would let you back into school in exchange for not having to pay their own attorney to defend them. I think if they are going to expel people for assault, then they should have a duty to being able to understand at least what assault is, as well as understand self-defense. You could also claim that you were discriminated against because of your gender. If it had been another guy, they would have not instantly taken the other side.
They can and would use the tape against you in court. You have the right to subpoena it before the court date so as to be able to adequately prepare your defense, but you have to exercise that right by taking action. Otherwise, you don't have the right to see it until trial, at which time you have the right to cross examine the presenter of the evidence.
You probably won't go to trial. The idea behind getting a lawyer is mostly to intimidate the other side, in this case the state, to accept a better plea deal. If you are going to actually defend yourself all the way through a trial, be prepared to spend at least $10,000 on an attorney.