Its all about planning. Pack your dinner in a cooler or lunch sack, just like you would if you were taking your lunch to work. I'm sure you have some time between classes, even if its 5-10 minutes that you can at least snack on your dinner.
Sometimes its easier said than done but you'll have to find a way to carve out at least 30 minutes 3 times a week for exercise. Wake up 30 minutes earlier than usual, leave for school earlier and walk in a park nearby or anything else you can come up with. You simply have to find a way to rearrange your priorities.
Good luck! You an do this!