As Bobs says, this seems to happen all too often and it's very stressful for everyone. My DD went through it as well about 18 months ago.
Yes, to some extent the girls have to stand up for themselves, but I found that there was a point where we had to step in and help. My DD had confronted the worst offenders, but it was really hard for her and didn't make any difference. Deep down she felt betrayed and didn't understand what was going on, her overall confidence, not to mention her schoolwork, was really starting to suffer. The final straw came when, in front of a whole group of people, they said "We're really sorry, we gave you a chance to be our friend but it hasn't worked out and so it's better for you if we stop trying." This was from people she'd known from Junior school, had done sleepovers and parties with etc etc etc.
Although some may be mature enough to take this sort of thing on, not every girl knows how to stand up for themselves against a group, however strong they may be. Just because they are no longer in primary school, doesn't mean that they are grown up enough to handle these things and it isn't a fault or a flaw on their behalf if it's hard.
The thing to remember is that this is bullying. It may not be physical, but it is bullying none the less.
Anyway. I emailed the deputy head of the school, detailed exactly what was going on and asked if she could help. She rang me back almost immediately and talked through the whole thing. She made it very clear the school had zero tolerence of this behaviour and she would act on it.
She did, with a vengence. First of all she confronted the whole group of girls who were causing the problem individually, and made it clear that the behaviour was unacceptable and threatened the ringleaders with expulsion. OK there was fall out from this, but it was all out in the open ... "we only said that to help you, it's not fair that we are being blamed etc etc etc", but DD just shrugged and told them that she didn't see it that way. Interestingly, once other form groups found out what was going on, lots of them all rallied around DD, which totally shocked her ... but she realised that other people didn't like this crew either and that gave her tremendous comfort. The fact that she had the support of the staff and she knew that she wasn't alone in school or being stupid but was being taken seriously was fantastic for her.
Within 24 hours we met the deputy head together and she suggested that if DD wanted to move form groups she could, and she was able to choose which group she joined.
She hasn't looked back since. It took a little while for her confidence to return completely, but it has. She has a new group of really nice friends now, her work is better than it has been for years and she is really happy. Interestingly, the girls who gave her a problem now talk to her quite happily again when their paths cross, though you wouldn't call them close friends.
So ... my advice would be to talk to the school if this goes on any longer. It needn't lead to drastic measures if that's not right for your DD ... but I believe very strongly that it can help a lot.
Hope things get better for her vvv soon.