There may be legal grounds for charges to be filed. Only your local prosecutor has the authority to file a case (no private attorney can prosecute a criminal charge) and usually the conduit to get to the State is through the police department. If you choose to follow up, they are the first people i would call, followed by the local prosecutor (District or State's Attorney) if you get no satisfaction from the police.
There was a crime committed at the school campus thus the school or local police should be involved. I would talk to the principal about this matter and tell them that you want charges filed. The police should be called. If not, you call them yourself. (At a minimum, the assaulting student should be sent to alternative school, away from your child, and kept out of your child's classes for the remainder of the year.)