WEll, there was a thread about this earlier, but it died. I say that you should argue with logic. "Why is it stupid?", "Why is it *insert whatever they say about ti here*?" Then, take the offensive like...
"You just don't like it becuase everyone else says it did. So you just follow the crowd and can't think for yourself? Is that it?"
They counter verbally with a personal insult:
"So you can't think of a way to counter that, so you spit out random insults. Very smart"
A logical counter is difficult to come by. They hit/punch you, take the blow in an obvious area, so when a teacher notices it, identify who you got it from. They can't say you're a dobber, seeing that you didn't go straight there. But the most important thing is to show that it isn't getting to you.