This is my first thought too. If they see someone commit a crime on school grounds they have an obligation to report it to the SRO/Police.
But ignoring this, here are two questions you should ask if you choose not to press forward. 1.) What will you say to the parents of the child that this girl cripples or kills the next time she does this...because we know she will if there are no real consequences. 2.) What lesson are you teaching your son if you don't. Someone attacks and assaults you. It requires going to hospital/doctor, but you were very fortunate. Hide it and cover it up, and protect the assailant. Next time it happens make sure to stay quiet and just take it. And also the next time. It will end when you grow up and THEN you should do the right thing and go to the police?
We all want to look at this about "Kids being kids", but that is not what this was. It involved kids and happened at school, but those things should be ignored when assessing the situation and actions.
I tell my kids "It's never too early to do the right thing. Sometimes it is harder and more inconvenient in that moment, but taking care of it early before it grows/festers is always the best course of action."