Mark Haddon 
Boom! 

Boom! Is a roller-coaster read with spaceships, spies and mortal danger!Jim and his best friend Charlie weren’t looking for adventure, when they decided to bug the staffroom . . . But then they overhear their teachers speaking in a secret language and it’s too late to turn back. And now they have to explain to the police, the headmistress and their parents where they’ve been for a whole week – without mentioning the aliens.