Easily my second favourite Lois Duncan. It's such a great concept and is so eerily spooky, with the added bonus of being in an exclusive boarding school in the country (written back before it was a cliché and was instead just plain old cool).
As always with Lois Duncan, it's an easy read with a strong female protagonist and a dark mysterious feel that will make you want to read with the lights on... but it won't give you nightmares.