Title: Nightmare Hall: The Voice in the Mirror
Author: Diane Hoh
Published: November 1995
Publisher: Scholastic Paperbacks
Annie is terrified when she learns the truth about her friend, whose split personality includes an evil half that believes every girl he meets is the reincarnation of a girl he once murdered, and Annie may become his newest victim. (amazon.com)
The plot alone sounds a bit schizophrenic. I like it. Just not the mirrors. I hate mirrors. Shit lives on the other side of that reflective surface.