From my experience with school bullying it was an entirely social problem. If the rest of the classroom agreed that you shouldn't be bullied then there were few ways for a handful of kids to go against that consensus, and most of them would backfire on the would-be bully.
Of course, if you're young and you have no friends, good luck getting your peers to think you're worth defending.
The one time I got attacked, one of the top three popular guys in the class went berserk on my attacker. This happened in grade school and the next aborted attempt at bullying wasn't until the end of middle school by someone who had transferred in later on.
Of course, if you're young and you have no friends, good luck getting your peers to think you're worth defending.