If you were ambushed and seriously beaten, then I would expect the school to expel the ringleader, suspend the others and get the police involved. One-on-one in school is one thing, but once they start planning ambushes they have gone over the "child" line.

I think the problem is that there aren't any perfect solutions to bad human behavior. Sure, this kind of retaliation can happen but that doesn't mean it was wrong to fight back in the prior incident where one doesn't have this foreknowledge.

Ender's Game comes to mind. Win. Period.