To quote Norman Stanley Fletcher, "don't let the b*****ds grind you down." Devilwoman, Luna-tic and Commonsense are quite right - these girls are insecure and not a little stupid. They feel they have to join in with the taunting because otherwise they may be shut out of the 'herd' (because humans have an instinctive, animal need to be part of a society, for survival reasons). You, however, are better than that. You don't need them, because you have friends elsewhere (here, for a start) who really don't care about the lies people are telling. Lose yourself in what interests you - sport, music, arts, whatever. It sounds cliched, but once they see that you really couldn't give a toss, they will leave you alone. It may take weeks, or even months, so just be patient.
Like Commonsense, I had something similar. It's been heartening in recent years to learn that the ringleader ended up cleaning floors for a living (after a grammar school education), whilst I've ended up relatively happy and fulfilled. I've also recently made the acquaintance of (as opposed to just meeting) a very well-known broadcaster who was bullied (because that's what this is), and I have a feeling this girl would be very jealous indeed if she knew this. After all, I was the uncool saddo she could treat like dirt whenever she liked, because she was my only 'friend'. Yeah? Up yours girl!
Dejucrazy, your day will come!! :-)