She's Not What She Seems — Francine Pascal

Shakespearean tragedy!
Jessica Wakefield has landed the starring role in Sweet Valley High's production of Macbeth, and suddenly she's too busy for anyone except Paula Perrine, a timid new transfer student who idolizes her. At first Paula spends all her time fawning over Jessica and helping her prepare for her role. But before long, Paula seems to be better friends with Jessica's crowd than Jessica is.
Jessica is pleased that her new friend is coming out of her shell, but when Paula falls to tell Jessica that she's been made her understudy, Jessica wonders if Paula is trying to steal not only her friends but her role as Lady Macbeth as well! No one else, not even Jessica's twin, Elizabeth, believes shy Paula could do such a thing. Can Jessica convince her friends of the truth before it's too late?