Yeah so if you punch someone in the face when all they did was verbally provoke you... you can still be arrested for assault. If the police get called they won't just let you go because you said 'but they were mean first!' Many of these places have their hands tied, though schools a bit less so. Bullies and trolls know the rules, they know the laws that govern the space they are acting in. They know where the line is so they can toe it just right so they can't actually get in trouble, and as already stated 'but because they did something' is not a defense for escalation. And that is what it is, it isn't self-defense, it is escalation. If you fight back toeing the same line it'll be less likely to get reported or acted on if reported. The moment you escalate it is on you.