Girls ar much meaner and more sly than guys when it comes to bullying.
There is agreat boook called Odd Girl Out that I highly recommend if anyone's dealing with this.
Thank heavens our daughter who was the focus of bullying not once buy twice had a school change that wound up separating her from the girls. She then chose to attend a performing arts high school that once again saved her from this particular group of girls.
Talking with teachers and others these are usually the popular girls, their parents are very active in school so their daughter's actions are viewed through different eyes.
The most important thing is recognizing it for what it is. Bringing it to the school's attention, even if they don't believe you your meeting will be documented in the records for future reference. Support outside of the arena is also very very important.