The Cheater, Volume 18 - R. L. Stine

Carter Phillips is under so much pressure to do well on her math achievement test that she gets Adam to take it for her, in exchange for one date. But Adam wants more than a date--much more. Carter has to do whatever he asks, or he'll tell her secret. As his control over her gets more and more unbearable, she wonders in desperation if murder is the only way out. Under pressure to perform well on her math exam, Carter Phillips persuades math whiz Adam to take the test for her in exchange for one date, but one date is not enough for the dangerous young man.