File charges. Force the school to act. Even if nothing ever comes of it, give the parents a big enough scare to reign their little monster in and so the school takes it more seriously, but if you threaten then you must follow through. I recently found out my 13 yo son was being bullied in school. Last week his bully got away with punching him in the face 7 times total over 3 days and choking him all while in English class and the teacher never saw it. I notified the school of everything. When I didn't hear back from them I called the principal back and let him know in a message I contacted an attorney and was pressing charges. I didn't need to say "and the school is liable". They got that part loud and clear. The boy is being expelled, but I'm still going forward with the charges.