Murder on the Blackboard - Stuart Palmer

Schoolteacher sleuth Miss Withers finds a dead body during detention in this mystery starring "one of the world's shrewdest and most amusing detectives" (The New York Times).

Anise Halloran is young to be teaching school, and much too pretty, but third-grade teacher Hildegarde Withers is not the sort to condemn a coworker just because she wears high heels. When she overhears nine-year-old Buster Jones spreading rumors about Miss Halloran being sweet on the principal, Miss Withers orders the schoolyard quarterback to write discipline on the chalkboard one hundred times. Anise Halloran stays late after school, too. In fact, she stays forever.
Miss Withers finds Anise in the cloakroom, her head bashed in, and her high heels strewn across the floor. She sends Buster to fetch Inspector Piper, the hard-nosed detective whom she occasionally assists with murder inquiries, but by the time he arrives, the body has vanished. There is a killer inside the elementary school, and Buster Jones is not the only person whom Miss Withers will have to teach a lesson about discipline.