The Demon Headmaster (The Demon Headmaster #1) by Gillian Cross

'I can see that you are not yet accustomed to our ways,' said the Headmaster. 'I hope you are not going to be a person who won't co-operate with me.'

On the first day at her new school Dinah realises that something is horribly wrong. The children are strangely neat and well-behaved; they never run when they should walk and they never raise their voices. They even work during playtime. What makes them behave this way and why does Dinah find herself conforming?

The answer is fear -- but what is the secret of the Headmaster's control and why does he exert it? With the help of her foster brothers, Lloyd and Harvey, Dinah is determined to unravel the mystery of the sinister Headmaster.