It really depends on you. I usually eat something (breakfast, toast, bagel, etc) before each class because I am always hungry.
I also drink water before I go and I take 2-3 bottles of water with me if it is hot out or 1-2 if it is cooler out. You may want to take a sports drink if it is really hot out and your class is more vigorous. Sometimes eating a small amount of food with salt (handful of pretzles) to help you retrain more water while you are at class.
I guess you should just see what works for you. I used to prevent from eating before class, but now I find that if I don't, I get hungry and sometimes I feel a little out of energy or lethargic without it.