The Substitute — Marilyn Kaye

Amy never knew whom Mr. Devon worked for--the organization that's after her, or some other secret government agency. All she knew was that he seemed to be her ally. He always appeared just when she needed help or vital information. But ever since Mr. Devon's death, Amy has relied on her own smarts to survive. And she'll need them more than ever now that a new substitute teacher at school has it in for her.
Ms. Heartshorn has a reputation for being demanding. She doesn't tolerate fools or careless work. And her feedback can be . . . well, a little harsh. Amy hates her. But when she learns why Ms. Heartshorn is really at school, Amy totally flips out.