The Cat Ate My Gymsuit by Paula Danziger

Marcy Lewis is bored by school, she knows she's never going to be thin, and her family life's a nightmare. Then Marcy meets Ms. Finney, an English teacher who will try anything in the classroom. Finally, Marcy's life in and out of school begins to have a purpose. But when Ms. Finney is suspended, Marcy knows she's got to take a stand, no matter the price she must pay.