The Demon Headmaster: Mortal Danger by Gillian Cross
When Lizzie's school gets a new head teacher everything seems great. The pupils are encouraged to do the things they really love, experts are brought in to inspire them; there's even a competition to win an adventure trip of a lifetime. Lizzie and her friends can't believe their luck! But then Lizzie hears a voice. A voice strange but familiar. A voice that says, 'I will obey all your instructions.' And that's when things start to get dangerous. . . Fast-paced and action-packed adventure, written by the award-winning Gillian Cross.