Out of Bounds 2003 on netflix

Louise is an alienated boarding school student in the midst of a hot and heavy affair with the husband of her headmistress, Veronica. Suspicion and passion hang in the air of the isolated campus until Matthew suddenly disappears. Fearing the worst, Louise struggles to uncover what happened to her lover but is foiled at every turn by Veronica, who begins to take a sadistic glee in the psychological torment she inflicts upon her. When a mysterious woman appears around the campus, Louise becomes convinced that she is being framed for the murder of Matthew, or even worse, being set up to be the next victim. Alone and friendless, Louise must keep herself alive long enough to uncover the truth of Matthew’s disappearance.