Violent bullying generally requires a violent response. It's unfortunately true.

My then 8-year-old son was getting punched by another boy in his class on a regular basis and the teachers couldn't do anything about it because they "didn't see it." I told him that if anyone ever hits him, pushes him or punches him again he has my permission to hit back commensurately just as hard and he would suffer no consequences - even if the school wanted to suspend him.

So the next day he got punched and he punched the other kid back in the stomach. It's been 2 years now and that kid never hit him again.