Claudia and the middle school mystery

For once, Claudia's studied really really hard for her math test. And when Claudia gets her paper back, it has an A - on it. All right!
But then, Claudia's math teacher calls her to his desk and accuses her of copying off of Shawna Riverson's test!

Claudia can't believe it. Neither can the Baby-sitters. Claudia would never cheat! And they're not going to rest until they help