Anthony Horowitz 
Groosham Grange [PDF ebook] 

Sent to Groosham Grange as a last resort by his parents, David Eliot quickly discovers that his new school is a very weird place indeed. New pupils are made to sign their names in blood; the French teacher disappears every full moon; the assistant headmaster keeps something very chilling in his room… What’s the meaning of the black rings everyone wears? Where do the other pupils vanish to at night? Most important of all, how on earth can David get away – alive?