Pete Johnson: The Phantom Thief [1998] paperback

You are in danger. The first time Alfie sees the boy, he appears as if from nowhere in the school detention room - the 'padded cell'. There's something odd about him and Alfie is definitely not pleased when he realises the boy has nicked his new jacket. But a ghost? His new friend Sarah is convinced - so convinced she is prepared to ghost-watch with him. That's when the warning message appears, scratched out on the blackboard by a phantom hand ...