I've heard a number of people, predominantly but not exclusively women, over the years remark on having nightmares about being back in school facing some academic calamity, such as an exam for which they are totally unprepared. For instance, Amy Welborn and most of the commenters in this thread; for instance, my wife. I, on the other hand, have never had such a dream. My wife was a straight-A student, while I'm a lifelong under-achiever in nearly every way. So I guess this is my little compensation.

On the other hand, for many years I had a recurring dream about nuclear war, which is almost as bad.