if i was you, i would just start making them feel quilty. like say "u guys are really mean u know that? she can't help the way she looks and if u have a problem with that, fine. but don't take it out on her." and just say stuff that would make them feel quilty. they'll eventually stop, but if not, ignore your friend and tell some one that is an adult close to u or someone u can trust. like the school counsler or your parents, maybe even a big sister. i know she says she will hate u, but in the long term, the teasing might stop and she will feel better and some day thank u for it.