Mrs. Malory and the Delay of Execution — Hazel Holt

When a schoolteacher at a prestigious English prep school dies suddenly, Mrs. Malory gets shanghaied into being a substitute teacher. She's more than a little uneasy about facing a classroom full of know-it-all teenage girls. To her surprise, they are neither typical nor trying. But it's not long before Mrs. Malory realizes that there is something strange about the teacher's untimely death...and something even stranger about the eerily obedient students.