There are so many good ones, it will be hard to choose which ones to read aloud, but I have my favorites. In most of these books, the protagonist is on his or her own through some kind of disaster, and not by choice. The notable exception is My Side of the Mountain , a book already familiar to most seventh graders. I like to read the first few pages of a book called Save Queen of Sheeba , by Louise Moeri. It tells of the survival of a twelve year old boy and his 5 year old bratty sister, after a Souix raiding party attacks their wagon train.
Grisly details are sure to delight the seventh grade audience.
Another good read aloud is The Island Keeper by Harry Mazer. I read the part where Cleo has finally decided to leave her island haven and go back to the mainland she has run away from, when she finds her canoe smashed by a storm. She then must face the prospect of surviving on the island until the ice is strong enough for her to walk the 5 miles back to shore.