Troublemaker — Emily Chase

There's a troublemaker on the loose.

Toby is down in the dumps until Andy and Jane promise her a special party to cheer her up. But first they have to get their headmistress's permission. Then Andy has to devote every spare minute to a major term paper. And Gigi "The Worst Person in the World" Norton decides to throw a rival party of her own, with a truly nasty theme.

When Andy's term paper disappears and Toby is the victim of a cruel letter, the girls of 407 realize that someone is out to get them. Who is causing all the trouble? And how much worse can it get?