Oh geez. Do I know that feeling. Like Diane, I was picked on a lot at that age. I was girly and bookish and nerdy and unathletic, and there were a couple of tomboy girls who used to pick on me relentlessly. One of them used to come up to me, pull my hair (which was springy curly back then) -- and I mean pull it hard -- and say, "boing boing". Another liked to mimic everything I said in a nasty way, for some reason.
Imagine my surprise a couple of years ago when the hair-puller friended me on facebook. We're cool now.
I'm really sorry your daughter is going through this. My mom made sure to listen to me vent whenever I was upset about this kind of thing. She was a shy child herself, so I think she understood well.
I think girls that age are trying to figure out their place in the world, and in the process they can sometimes act like little cretins as their insecurities surface. It's the age when kids start sizing each other up and comparing, and while some of that is inevitable, not all of it healthy. It seems to be a typical age for old friendships to get shaken up and tested, and new friendships to form in their wake. The best friend I found after my 5th grade trauma year is still my best friend to this day.
I have a feeling it is even harder today, in the age of the internet and "reality" TV. It seems that people get loads of attention for awful, nasty behavior, and it's hard to shelter kids from experiencing the trickle-down effect of that.
Hugs to your girl. She'll be fine. It is a hard age, and the teen years are still to come. Buckle up!