Jag tycker att det var boken Horror High - New Kid på Blokken av Nicholas Adams.
Synopsis :
Elizabeth Henley can't wait to meet Amanda Mackenzie. They've been pen pals for a year, and now Amanda is coming to Cresswell as an exchange student. But when Amanda shows up, there's something very weird about her. Then a boy is found dead in the woods. Amanda's increasingly strange behaviour causes Elizabeth to be suspicious. After a second body is found, Elizabeth realizes that the girl who is staying with her is not her pen pal. Will Elizabeth's discovery make her "Amanda"'s next victim?