The Sister Switch (Candy Apple #11)

Candy Apple is a fresh, fun take on fiction for girls: a new line of single titles with pep and pizzazz targeted at the solid middle-grade reader.
Twins Andie and Caitlin are best friends but they couldn't be more different. Andie loves sports and hanging out with her friends -- Caitlin is passionate about music and loves learning at school.
Now that junior high is starting, the girls will finally get a chance to pave their own way. They can't wait!
But when Caitlin discovers she'll have to pass a fitness test or spend every day after school doing gym, she fears her year is ruined before it even begins.