Three 8 yo girls equal big fights. How can I help them learn to get along?

My daughters class had 4 girls. One doesn't like to play with the girls, leaving 3. One is my niece. My daughter and the other girl are both strong willed, assertive girls who want to be the "leader" of the pack. My niece is a girl who is happy to follow. However there can't be two leaders so there is always an odd man out. My daughter has been the odd man out for awhile now and its makign school miserable. When she sits down at lunch, the other girls move. She plays alone at recess becuase it hurts her feelings to be rejected when she asks to play with the others and they say no. Right now her best friend at school just might be the janitor. I've talked to the teacher and she doesn't know what to do about the situation, and I've talked to the mom of the strong-willed girl and she hasn't heard anything about what's going on (probably cause her daughter isn't suffering). What's a mom to do???