Well, that's the thing. Bullying is already against the rules. It was in pretty much every school I went to, and you go to a forgeted up school if it isn't.
The problem isn't (usually) the teachers, it's us, the students. The kid being bullied never told an authority figure, and witnesses just walked away.
As Night Fox said earlier in the thread, a rule doesn't apply if you aren't caught breaking it, and nobody is ever caught because of this stupid "snitching" mentality that we've created.