Title: Nightmare Hall: Last Breath
Author: Diane Hoh
Published: November 1994
Publisher: Scholastic Paperbacks
Summary:
A student discovers that school can drive you crazy in this entertaining twist on the classic thriller Gaslight. Cassidy has the perfect life . . . until things start to go really wrong. People are starting to whisper that she's losing it, that's she's gone over the edge. Now someone is about to end Cassidy's perfect life--with the perfect death. (fantasticfiction.co.uk)
This one sounds like it could be the best of them yet. I'm a big fan of anything psychological, especially psychological horror. You can't get anymore horrifying than your own mind.