Legal Question in Civil Rights Law in California

How are you going to prove it was self defense. Also, perhaps even if it is in self defense, your son cannot use a larger amount of force than that which he was threatend with.

You really have to ask that? Schools are legally obligated to investigate and take disciplinary action against everyone participating in violence. Everyone involved usually says 'I'm innocent'. Without credible witnesses to corroborate his story, his word won't count for much. Hire local counsel if you are willing to spend the money to have a 'defense presentation' done to try to minimize the damage here.