There's Someone Inside Your House by Stephanie Perkins

 

(From Amazon)

One by one, the students of Osborne High are dying in a series of gruesome murders, each with increasing and grotesque flair. As the terror grows closer and the hunt intensifies for the killer, the dark secrets among them must finally be confronted.

(My Review)

This book grabs you right from the very first chapter.  And it is not for the faint of heart - it is a high school book, but it is a bit graphic.  It was definitely a creepy story that kept me reading, and kept the suspense until the very end of of the book.