Cosmic by Cottrell Boyce

Liam is a 12 year-old boy who is very big for his age. So big he is not only mistaken for a teacher but is also able to pose as the father of a classmate. In COSMIC Liam winds up the only adult with a group of other 12 year-olds on a spaceship. In space. Liam has brought along his father's guidebook to dealing with teenagers and Liam uses the guidance of the book to help deal with the other kids especially when they are possibly lost in space.