Ian Harrison 
The Jiggery Code [EPUB ebook] 

Tom is sitting on the terrace at Beastleigh Hall when he looks up and sees the Sweeney-Todd gang pushing a huge weighty gargoyle off the roof while the headmaster is sitting directly below it. Even with the help of the Jiggery Stick, Tom has to move fast. He rugby-tackles the headmaster out of his chair, and they sit in a jumbled heap well away from the overturned chair, which has now been crushed by a ton of masonry. Can Tom stop the gang from killing the new headmaster, who is a friend to the Rats?
After he leaves school, Tom kills a gangster with the aid of the Jiggery Stick and is weighed down with guilt, but a chat with his grandfather soon solves the problem.
Let the mayhem begin.