I have the Wald folding baskets, and they're fantastic when you're using them, and a pain in the a** when you're not. They're mega heavy and the latch that holds the baskets up while they're folded always catches or rips the bags when I'm trying to take them out of the basket.

IMHO, if I were just going to be lugging around a backpack to school, I would get one of those plastic milk crates and put that on my rear rack. Sooo much lighter and easier to use with plenty of space. They're very popular on my campus