okay, this might sound a little far out.
I've had the same problem as you, and if the reason isn't bad air in class, then meditation could do the trick.
I allways feel clear and energic after a meditation session, it helps to keep your mind sharp, true story.
In this article they say you should meditate as many minutes that your age, so if you're 20 yrs, you should do 20 minutes sessions.
doesn't harm to give it a try:)