The Silver Chair (1990)

Eustace is sent to a horrible school, and finds a friend in Jill Pole, who's also running from the 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 Rillian. Together with Puddleglum the Marshwiggle, they must travel north across the mountains, dodge dragons and giants, and journey down into the earth itself to rescue Rillian from the Green Lady.