It sounds like she is having a horrible time, and it sounds all to familiar. My daughter and many of her friends seem to have gone through much the same sort of things in year eight and nine of all girls schools. My gut reaction is to say move her now, howevr I think that depends on what the other options are. In our case we didn't move our dd after much discussion because we felt that she would not necessarily interview well as her confidence had taken such a bashing ,and we were worried that would choose a school in a hurry because it had a place available and not necessarily because it was thee right school. We also felt it would thin be difficult to move her again for sixth form.
In the end we left her where she was, but, year ten bought with it so many changes that the bullying behaviour has drifted into the background. In the first instance the forms were all switched around, breaking up the groups that had formed in the first three years. This helped. However, the more obvious thing was that thanks to gcse option, the girls all move classes all the time throughout the day. Up til now she has had the same girls for all classes except magus, the only class that was setted. Now every class is different, and the girls that had made her life a misery just are not there all the time, and are themselves distributed across all the classes and so are less of a obvious group.
This is a long winded way of saying that I would council moving, and like everyone else I would strongly consider a mixed school; however if that isn't possible I would look at her own school and see whether the structure of her day will change dramatically next year.
In our case I thinker should have moved our daughter, but she is much, much happier this year, so lthough we havent done the right thing, it has worked out.
In our case the school also did little to help, despite the fact that a large number of girls were affected eby the generally nasty attitude in the class.