They shouldn't necessarily bully them back, but they should be taught to defend themselves. Outside interference may fix the problem, but more often than not, it really doesn't. That saying "the best way to destroy an enemy is to make him a friend" was from Abraham Lincoln, and those kinds of quotes are always ridiculously ignorant and cliche.
If bullying makes you a bully, bullying back makes you a bully too. Self-defense is not bullying.
Na ah, don't give them a piece, be sensible about the situation. No one should be treated like that, bully or victim. We are all people and they are probably bullying you as a reflect from their own problems. Treat them nice and comfort them and they might stop and don't be mean back