Jessi and the Awful Secret — Ann M. Martin

When Jessi's dance teacher asks for volunteers to help out in a ballet class for children, Jessi gladly signs up. She loves kids, and this will give Jessi a chance to get to know some of the girls in her own class better.

Soon, Jessi wishes she didn't know them so well. Because Jessi discovers a secret about Mary Bramstedt. One that could hurt Mary. Jessi knows she has to tell someone. But will telling the secret mean losing a friend?