Dreams.

Yes, I also sometimes dream about school, and the initial theme is usually the same: something like, there's still one last diploma to get or one last class to finish. And even though nothing bad or stressful happens in the dream, it's just that I have to go back to school again. So, I think the connection here isn't with specific events that happened there, but rather with the fact that my psyche is permanently scarred by the pressure to get an education.