Having recently just been through this exact same thing with my 8 yr old, I definitely agree with having a quiet word with the teacher. My DDs teacher was shocked when I told him what had been going on as he hadn't noticed anything. The following day he did a class talk about bullying and how it was unacceptable and if anyone was found to be bullying then they would be punished accordingly and the matter referred to the principal.
It seemed to do the trick and DD said that the girls let her play with them, she even had a couple of them over for play dates over the holidays. It turned out that the girls were scared of the ringleader and she had told them they weren't allowed to play with DD.
And if you feel that the teacher isn't dealing with it go straight to the principal.
I really feel for you and hope things get sorted soon