The Chronicles of Narnia: The Silver Chair.

Eustace is sent to a horrible school and finds a friend in Jill Pole, who's also running from bullies and looking for a place to hide. The two of them are magically transported from the garden shed into the magical world of Narnia, where they are entrusted with a task by Aslan: to rescue the king's stolen son, Prince Rilian. Together with Puddleglum the Marshwiggle, they must travel north across the mountains, dodge giants, and journey down into the earth itself to rescue Rilian from the mysterious evil that holds him bound there.