Girl at school keeps beating up my son

My son is in 7th grade and keeps getting picked on by this girl in the 8th grade. She has stolen his things. Beat him up and got people to gang up on him. I've been to the school about it. We've met with her parents and her parents actually laughed when they found out their daughter beat up a boy and said "That's my girl." The school gave her a two day in school suspension. He came home last week and she had given him a black eye and stole his backpack and glasses. My son's father thinks she just has a crush which is irrelevant. I'm not sure what else to do. So I told him I know I told him to never put hands on girls but he do not keep allowing himself to get beat up and if he ever has to defend himself than do that. Do not let people beat you up. Do not allow people to take your things. Regardless of gender. I hope I'm not giving him mixed messages and I am a not violent person but I dont want to keep getting bullied.

@G i spoke to the police dept about what happened and what I can do. What this officer told me is "Kids fight." And that I can press charges but not much will probably come out of it. This is what an actual officer told me