The Demon Headmaster and the prime minister's brain - Gillian Cross

Octopus Dare is the new computer game at school - everyone's playing it. But only Dinah is any good at it. Soon all she can think about is Octopus Dare. She wins a place at the grand final, and she's over the moon. But there's something strange about it all that worries Dinah - so her friends come with her to the final. Before long they find themselves in trouble...facing their old enemy, the Demon Headmaster.* Gillian Cross has won the Carnegie Medal, the Whitbread Children's Novel Award, and the Smarties Prize, and is a highly-acclaimed author. Sales of her books exceed one million copies.