Demon Headmaster, The (Vintage Paperback) by Gillian Cross

Dinah moves in with the Hunter family and starts going to the same school as her foster-brothers Lloyd and harvey. It's not easy, as they seem to hate her, and school is really strange. Pupils suddenly talk like robots and do weird things - even Dinah finds herself acting oddly. She's sure the headmaster has some kind of power over them, and is determined to find out more. But the Demon Headmaster is equally determined to stop her.