Best thing to do is ignore them as most replies are going to add fuel to the fire, because it's likely your reaction that they're entertained by.

Wit, however, is a skill like anything, and actually delivering a sick burn on the spot when it counts is hard, and takes time to be good at. (Timing and delivery are key, if you waver at all, even a good retort will flop)

What I would say is just continue being yourself and don't let the haters get you down, because it sounds to me like maybe you're focusing on the right things at school, and maybe they're not.

With a good attitude, a clear conscience and your head held high, you'll always come out better off than them in the long run.