Unfortantely this drama with the girls only gets worse. My DD was fine with kindergarten and all the girls were pretty friendly with each other, but by first grade, the cliques started, or girls started pairing up and being "best friends" and leaving other girls out. Some girls, for whatever reason, are cattier than others and do indeed decide that a certain girl is their friend one day but not the next. For kids who become very attached to their friends, it's very difficult.
My DD has overall been pretty laid back about the situation but started having a really hard time the second half of last year. She always seemed to be upset about somebody or something going on at school. We realized that she was having issues with her ADHD and the medication. When she was on Adderall and Concerta (not at the same time), she became much more moody and hypersensitive about these situations. Now that we have her on a different med she can come home from school and say "J and S didn't want to play with me, but that's Ok, we'll play another day." So much seems to depend on their psychological state in terms of how they can deal (and also maturity--DD is now almost 8).
The guidance counselor and school psych also go into each classroom regularly and do a "Second Steps" program to basically teach kids how to handle their emotions and be good friends to others, and I think that has helped to minimize some of the drama with the girls. I know it is difficult and wish I had some more specific suggestions. I have always advised DD that if someone doesn't want to talk or play with her, to just let it slide and find a different friend who does want to be with her. If the behavior with one particular kid is ongoing (i.e. they never seem to want to be with her), then she needs to question whether they are a friend and worth her time to pursue.
