Instead of trying to fit in a big exercise routine every day, split it into 3 10-minute segments. So 10 minutes in the morning, before you go to work, 10 minutes before you eat lunch, then 10 minutes after work, before school. Walk around your building, or even just march in place in an empty room or the ladies room. (Make sure you're safe-- night classes often don't have adequate security.) That might seem weird, but you'll feel a lot more weird if you gain 20 pounds. These mini-workouts might not seem like much, but it does add up. And on the weekends you can get in one or two longer walking sessions, or maybe go to the school gym.
Also make sure you get up and stretch when you're at the computer, at least once an hour. You may be at risk for carpal tunnel syndrome with all the typing and writing you do.
Stay away from the vending machines. Pack healthy munchies in zip-lock bags. Stay away from added sugar and fat, it just makes you more tired. Concentrate on lean protein, veggies, and whole grains. If you must eat fast food, Subway is much better than McDonald's. At this point in your life, you have much more control over your diet than your exercise. That's your first line of defense: low-fat, low-sugar, healthy. Good Luck.
Age 48, 5'6" Living with PCOS.
Start: January 1, 2002, 220 lbs., size 24
Current: 163 lbs, size 10 - 12, 39"-31.5"-39.75"
Goal: size 10 in Levi's 515 jeans.