Chain Letter

This book was written by Christopher Pike. Here's the description:
When Alison gets the chain letter signed "Your Caretaker," she thinks it's a sick joke. But then it becomes clear that someone, somewhere, knows about that awful night when she and six friends committed an unthinkable crime. And now that person is determined to make them pay.
One by the one, each of them received the chain letter demanding dangerous, impossible deeds. No one wants to believe that this nightmare is really happening, but then the accidents start. And the deaths.
Uncovering the stalker's identity seems to be the only option, but even that might not be enough. The caretaker has an apprentice who is even more frightening -- and out for blood.