Harrison, Lisi. The Clique. 2004.
This is the first book in the very popular middle school series. I would describe this as Gossip Girl for younger readers, and without the smut. It has lots of catty behavior, and lots of name dropping, but none of the sex and stuff like that.
Massie is a part of a group of four really popular girls at their very exclusive private school in Westchester County, NY. Massie, Dylan, Kristen, and Alicia. An old friend of Massie's father decides to move there from Florida and takes a job at the same company. Until they can afford a house of their own, the Lyons family will be living in the pool house. They have a daughter, Claire, that is also going into 7th grade. Massie tries to push Claire on her and Massie doesn't like it one bit, especially since she is a bit of a dork. Claire wants to be a part of their group desperately regardless of the fact that the girls want nothing to do with her.
Recommended for girls in the middle grades that like gossip-y type books.
Other books in the series:
Best Friends for Never
Revenge of the Wannabees
Invasion of the Boy Snatchers
The Pretty Committee Strikes Back