L.,
I had the exact experience as you and my daughter is now twelve.
I honestly think it is a healthy part of growing up - I say this now. On the side lines I felt helpless and at FIRST, went to talk to everyone, but nothing happened. My husband - her father - gave us some excellent advice that some one on this site said as well -when the two of them or alone no one will be there to stop anything.
Her dad turn to our daughter and said you have to stand up for yourself. And she was like, "Is that ok?" Her dad was like yeah! He said, If she says mean things when no one is around or listening - say mean things back - no curse words etc., If she pushes you or hits you (yes, pushing was involve, but no one ever saw it) push her back harder!
In fact when she is alone go up to her and say if you don't start nothing there will be nothing - and if you DO start anything I will finish it. He even made the fist in hand jesture. Sit where she sits,stand where stands - don't hide from her.
She said, "But dad, what if I get into trouble?" He said, "Who are they going to call?" She said, "My parents - you guys." "And we're saying it is all right, right?"
Boy did my daughter stand up for herself - she even made the mean girl cry, by sharing candy with the other girls and not this girl. I stepped in - she was becoming a mean girl now - and told her standing up for yourself is like a sword you have to be carefull how you wield it. That year she learn how to stand up for herself - without being a bully herself- it was great lesson for her.
Also, she befriend the girl (they were so much alike) they are still friends till this day - really, really good friends. Also, my daughter is very popular. I was making her into a victim, her dad empowered her and we both taught her how to stand firm, but be fair and kind.
K.