It would be unusual to have been on Adderall for 6 years and then to have it be the cause of your feet swelling. Plus, that is not a side effect that I have ever seen.
I think it is much more likely that during class time you are sitting for long periods of time and that is what the contributing factor is. There is a pretty simple way to tell. Take Adderall on a Wednesday (when you are not in class) and see if you have the swelling. or don't take the Adderall one day when you go to class.
I am not positive, but I think that if you find the Adderall is not the problem, then you should consult with a doctor. Swelling usually indicates poor circulation and you need to find out why.
I think that trying to move more during the class time will help. At break time, get out and walk. Perhaps, pick a seat in the back of the room so you can get up and move around a bit half way through (I would probably let your instructor know why you are moving). And, I think, that things like compression socks or leggings can help....but check that out with your doctor.
But, do make an appointment. Something has changed and you need to find out what. I hope this helps. Best wishes. Oh, and I would really like to know what you find out.