The Demon Headmaster by Gillian Cross
When Dinah is fostered by the Hunters she thinks her biggest problem will be fitting in with her foster-brothers, Lloyd and Harvey. However, once she starts at her new school it's clear that there's more to worry about. All the children, apart from a handful including Lloyd and Harvey, are too well-behaved - robotic almost - and oddly keen to please the creepy headmaster. The three children set out to discover the nature of his influence over everyone . . . but then Dinah finds herself saying and doing things she has no power over. Soon they uncover the headmaster's wicked plan. Controlling the school is just a practice run. He has set his sights on dominating the entire nation! The children must foil him before he succeeds . . . but with Dinah under his spell they've got a challenge on their hands. Great fun and just a little bit frightening, Gillian Cross's beloved classic still holds readers under its hypnotic spell, even after thirty years of being in print. An accessible page-turner, The Demon Headmaster is impossible to resist.