Some things you of course couldn't study in a dream. You couldn't study new information, but you might be able to sort through old information, to test yourself on things you've already memorized to strengthen the memories, or practice problem solving. The problem is, as I undersatnd it, people usually have a difficult time remaining in their dreams if they aren't immersing their senses in the dream world. So studying in a dream might prove difficult unless you can come up with some creative way to visually represent what you're trying to study.