Why do i keep dreaming this?

I always have dreams that im back in highschool. I graduated in January 2010. A few times a week ill have dreams that im back in school suddenly. But in the dreams i know i have already graduated and keep telling teachers that im not even supposed to be there but then theyll be like 'oh, well you missed this ignment, and you missed too many days and didnt get the credit'. Wtf? Why do i keep having these dreams? Aha, not really a big deal, im just curious. I hated school and thats why i decided to graduate early.