M.S.
I am so sorry... and totally understand your frustration... I saw this even when I taught kindergarten. It's the very reason I am scared to death of having a girl in this day and age... Boys fight and it's over.........With girls, it's psychological warfare....... (and when they're older they use texting, Facebook, etc.... to do it too)
Unfortunately, cliques don't end in school... they exist in adults too... (same goes for bullies) I know it's tempting to try to rescue her and protect her from these girls. Talking to other parents rarely helps... (especially if the parents talk to the daughter, and then they take it out on your daughter as 'the squealer...') I think the best thing you can do is give your daughter strategies to deal with these girls, build her confidence, and get her in some activities to build her confidence... She'll be well-equipped for dealing with these types of girls (and women) in the future.