Ænigma (1987)

Kathy is an outcast at an elite New England boarding school whom lands in a coma resulting from accidentally being hit by a car during a cruel practical joke against her by several car loads of her tormentors. A little later, Eva, a new arrival to the school, takes over Kathy's old room and begins socializing with some of the girls responsible for Kathy's condition. It turns out that Eva is a pawn under the control of the comatose Kathy from her hospital bed in seeing revenge against those whom did her wrong, while Kathy's physician, meets and begins dating Eva whom also targets his latest girlfriend.

IMDb: https://www.imdb.com/title/tt0092516