The Teacher by Freida McFadden

Lesson #1: trust no one

Eve has a good life. She gets up each day, gets a kiss from her husband, and heads off to teach math at the local high school. All is as it should be. Except...
   Last year, Caseham High was rocked by a scandal involving a student-teacher affair, with one student, Addie, at its centre. But Eve knows there is far more to these ugly rumours than meets the eye.
   Addie can't be trusted. She lies. She hurts people. She destroys lives. At least, that's what everyone says.
   But nobody knows the real Addie. Nobody knows the secrets that could destroy her if people at the school found out. And when Eve discovers the shocking truth, Addie will do anything to keep her quiet...
   From Sunday Times bestselling author Freida McFadden comes a story of twisting secrets and long-awaited revenge.