I have an 8th grade girl. We have witnessed first hand how MEAN some of the girls are at this age. You have received some good advice.
The girls are establishing their "place" within the cliques, etc. My daughter is in cheer and and you can obviously see 3 cliques within the group of about 20 girls. At our school you have the preppy's, skaters, and nerd/geeks
Our theory here at home is: She is going to be around all types of people as she grows up and she will have to learn how to get along. When she is with friends (or non-friends) at school or home, the interaction between the girls is like rough stones rubbing together....eventually they smooth out.
My daughter is one that has friends in the skater group and prep group and she is not really associated with a specific group. The stories she tells me about what some girls are unbelievable. Some of them are so spiteful and downright mean.
She is stressing out now over the halloween party she is planning because she knows she has to keep some of the girls separated, therefore, some are not invited just so the party can be a success.
As for self-esteem, I strongly agree with martial arts. My daughter achieved her black belt 2 yrs ago. The discipline, structure and self esteem works wonders. She uses her black belt knowledge in her daily life. One of the traits she tells me a lot is perserverance. She has a poster with that on her wall to remind her keep on going when things get tough. Her other one is "Never Give Up."
I admire you for being there for your daughter to help her through this. Middle school is tough socially and these kids need mom and dad more than ever.
TF