The Friendship War by Andrew Clements.
Grace and Ellie have been best friends since second grade. Ellie's always right in the center of everything--and Grace is usually happy to be Ellie's sidekick. But what happens when everything changes? This time it's Grace who suddenly has everyone's attention when she accidentally starts a new fad at school. It's a fad that has first her class, then her grade, and then the entire school collecting and trading and even fighting over . . . buttons?! A fad that might also get her in major trouble and could even be the end of Grace and Ellie's friendship. Because Ellie's not used to being one-upped by anybody. There's only one thing for Grace to do. With the help of Hank--the biggest button collector in the sixth grade--she will have to figure out a way to end the fad once and for all. But once a fad starts, can it be stopped?
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