It's quite a common scenario in teenage cliques.. sounds like ur
friend there is still at a growing stage, insecure about her own
personal image and dealing with alot of changes in hormones.. it's
a confusing time of her life. That's why she teases the rest of u
to make herself look better (weapon of mass distraction).. and
since nobody actually dared to tell her off till you came along,
she thinks her behaviour is acceptable (or even encouraged).
2 things - you can either walk away, or apologize to her. Try
talking to her one more time, after things have cooled off a bit..
listen to what she has to say. Whether u extend ur friendship to
her again will be entirely up to you..
Personally I'd go for the whole apology thing to cover my own back
(You do not want to be the star of "House of Flying Daggers", trust
me)! .. but remember to state why you are surprised she would mind
since she does it every day to others... at least this way, no one
will be able to fault you! You might even earn yourself some
unexpected admirers at school..
Keep things simple and soon you'll be an expert in handling these
situations 
