My DD at that age made some choices that resulted in similar exclusion. Yes, partly because girls ( and parents too I was saddened to learn) can be vile, but she also had to acknowledged the part she had played. We gave her the choice of leaving (we were so angry and upset on her behalf my heart just wanted her out of the situation) but you can't always run away from your problems.
It was her choice, she stayed. We discussed what she would do to re-establish herself. It was a plan, with goals and objectives. That did not include sucking up to the really nasty bitches in the group ( I hate to use the b word but there were some) or trying to get in with the popular girls. It was about thinking who was decent and kind and being authentically friendly to those girls. Being a loyal and good friend, fun to be with and honest.
She and I also joined a new social activity together, mixed ages so she could make out of school friends, with my support.
2 years later. She has just been peer selected as a year group leader, and last week one of the girls spontaneously told DD that she is one of the most popular girls in the year because she is the only girl who does not say anything about anyone behind their back that she wouldn't say to their face.
If moving is the right thing for your DD ( and it sounds like it may be) my advice is to really talk to her about relationships.
We talked about the difference between unconditional love that families share and the love and respect of friends and boyfriends that is based on trust, honesty, shared values, aspiration and interests. You can't take it for granted and you must be able to express your self and not be taken for granted either.
We did acknowledge the nastiness of others but we did this in the context of explaining that the only person's behaviour she can control or be responsible for is her own. Focus there. No point dwelling on what X said or Y did, that is their problem for their conscience. If you & she can articulate what is bad form in the way others behave she can learn from that. Our DD decided that she would be sure not to get sucked into bad mouthing others or get involved in their teenage dramas, she decided she would feel less stress.
And Help your DD to understand the value of genuine, good & kind people to be friends with...( not necessarily the in or cool crowd) who will accept her for herself ... Now My DD's main friendship group is an eclectic mix and much more interesting for it.
Good luck . It will all work out in the end.