T.M.
How sad and unfortunate for your little girl!
I'd contact the school and find out if they have an anti-bullying policy. Our school does and it extends to their time on the bus.
However, short of that, I think this is one of those really, really hard life lessons we try to protect our kids from whenever possible. It's hard to accept that people may not like us regardless of how kind we are to them. It's hard to feel shunned and left out. But, not everyone in life is going to be nice, give hugs or sing koombaya by the campfire together, right? Maybe you can help her channel her feelings by doing something creative, like drawing how it feels to be bullied or even having her write a journal or book about her experiences. She can indulge in a fantasy of "What if everyone treated others with kindness and respect?", she can role play with her dolls, etc.
Also, I'd consider enrolling her in some extracurricular activities like girl scouts, swimming team, etc. to allow her the chance to make friends with people outside the neighborhood and even outside her own school. She'll see that she can be accepted by others regardless of how these girls are behaving.
Good luck : )