Blubber by Judy Blume.

In a fifth grade class that could be anywhere in the western world, Wendy, the class alpha-female, uses her power to make life miserable for Linda---aka Blubber. All the girls allow it and encourage her, either actively or passively. Jill learns the hard way that wrong is wrong-- it doesn't matter who the players are, and before she knows it, the shoe is on the other foot.
Judy Blume is amazing-- everyone knows that. Written in the voice of Jill, it is hard to believe a 10 year old did not write this herself--the language and characters and the situations are so authentic. Blume never preaches, but lets Jill tell a story as it occurs-- as the days pass we see her grow and start to question what is happening-- she moves from discounting the victim to feeling compassion for her and begins to challenge the Alpha's hold on their 5th grade world, only to find herself-- as so many other girls have-- the new victim. It happens so easily. This kind of first person growth is always a style i enjoy. It reminded me of the LOUD SILENCE OF FRANCIE GREEN by Karen Cushman-- which i also recommend,.
Loved it, loved it, loved it. Highley recommend.