The Curse of the Black Cat — Carolyn Keene

Exclusive Waverly Academy looks like a medieval castle, but the horrors there are up-to-date. With minor incidents and cruel pranks running rampant, and each incident accompanied by a curse note from the Black Cat, soon everyone is spooked -- even the teachers. At the headmistress's request, Nancy and George go undercover as teaching interns, and it isn't long before Nancy herself gets one of the menacing notes. Who is the mysterious Black Cat? An unhappy student? Or is a more sinister force at work?