How do I press charges against a minor?

My son was stabbed on his hand in school by another student. I talked to the school's dean and the officer (works at school) that was there at the time. They both said that the student would be suspended and following the suspension, an expulsion would follow for bringing a knife to school and using it as a weapon to stab my son on his hand with it. Well, it's been 2 weeks, the child was suspended. Now he's back in school and my son sees this kid every day in his lunch class. The kid has even sent messages through his friends to my son, "Not to press charges against him". I've contacted by son's school today but couldn't speak with the principal. She was busy , so I sent her an email. I didn't want to talk with the dean again because he has already lied to me about the kid's alleged expulsion. What do I do in this case? I'm afraid that my son maybe in danger, by going to the same school that his attacker is still attending and he sees every day and my son has been threatened to not file charges against this kid, yesterday.