I'm sorry you and your daughter are hurting. It would be so hard to walk your child through that. =(
I am a teacher and as much as we do promote respect for each other, there will always be drama between kids as they attempt to seek out a social "pecking order" - not unlike those forged by adults. It's human nature and because kids are immature and ego-centric, it can translate into bullying.
I don't know ANYONE who hasn't experienced this to some degree. Please do not switch schools over this - there will always be someone that your daughter will need to learn to get along with. Try to focus her - empower her! - on the things about this situation that she can change. We can never control how others will treat us, but we can control our responses to them. We can also control where we focus our energy. Try to encourage your daughter to expand her circle of friends to include those who are kind and considerate. If she shows indifference to this group of mean girls, it will take the wind out of their sails - their actions aren't eliciting the desired response (no fun!) so they will either change their tune or simply move on.
I am a middle school teacher. There are SO many kiddos coming up who have no idea how to interact w/others, communicate, and solve their own problems bc they have either been allowed to run from them or because mommy and daddy have always gone to bat for them. These are necessary life skills. While the parents' intentions are good, they have robbed their kids of the opportunity to develop skills in social situations. They have not empowered them to be socially self-sufficient or handle rejection. As a result, the kids meltdown or shutdown - emotionally stunted.
You have a golden opportunity to teach here, but it is not easy. Growth never is! This will require courage of both you and your daughter but it's well worth it. It's so wonderful to see an adolescent evaluate these situations later and decide - with confidence and clarity - that those people are not worth their time and emotional energy. They are healthier, happier kids.
Good luck, mama - eyes on the prize!