Pete Johnson 
Phantom Thief [EPUB ebook] 

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 . . .