From xkcd #557:
FULL ACK!
In the first years after my school graduation, already attending university, those dreams developed the situation that graduates are compelled to revisit school for one month every year to refresh a basic knowledge in various subjects—a not too weird idea after all. I went through situations like not remembering the location of our class room, teachers being upset about me not having shown up for long, or having hopelessly missed several exams. Through the years those dreams had to cope with my consciously strengthened memory about my school graduation and about that such a revisiting month simply doesn’t exist. However, a geeky brain like mine is still able to form a new plot.
I think I got rid of those dreams with my university graduation, having settled the idea of not having to attend unpleasant exams ever again. But I probably enjoy being a science/math/computer nerd way too much and should deal more with average daily knowledge about politics, economy, society and culture.
A reason why those dreams come up at all might be that stress and pressure at school are much more formative than they will later be in real working life.