I had tons of dreams like that as an undergrad (often involving classes in the middle school library - where no classes were held - and college classmates.) I've only had a few since starting grad school, though. They usually involved a very college-based situation, having a final in some class I hadn't attended all semester or forgetting when the final was. I think we go back to high school because it's such a constant setting. My college classes could be in several different buildings and take place at different times of day, but the middle school library was really distinctive-looking.