Fighting them won't work unless they physically harm you first. I don't know what your parents views are on fighting but when I was younger my father always told me if someone were to hit me first, by all means go ahead and kick the shit out of them. Which in the long run could be a win-win because now they won't mess with you, and your parents won't be on your back for fighting at school. In school talking to them won't work since the majority of kids like that only have enough brain cells to breathe, so I think talking to someone at the school might help. It'll look lame because it'll be looked at as "tattling" but that's the only mature way of handling it. That or have you and your friends step toe to toe with them and threaten them back. People that prey on one person more likely than not won't bother when more people are involved.