Get a standing desk. Do your homework whilst standing. This will help burn a few calories. Every half hour or so, take a ten minute break to jump rope, lift some weights in your room (you can fill empty milk jugs with sand or water to make weights if need be) do some core exercises or some stretching, get back to your homework. Walk, if you can. Take a longer route home from the bus stop or if it's an option, walk to and from your classes. Not sure if you're in university or not. If you must sit, get a balance ball and sit on that; this will help start to strengthen your core as you sit. Walk and read, if you can. If the book isn't too heavy carry it around with you around the house, around the block, etc.
Time managment in school is soooo difficult. I'm on my last semester now, and with a full time job, full course load and the gym it can feel overwhelming. I've turned the gym into my "me" time. It honestly really helps with stress and relaxation.
If the gym is too difficult to make it to (i work at my gym as a coordinator and its within walking distance of my school/home so that makes it easy), kel's suggestions are really good and can make a big difference. My older brother lost 60 pounds just walking home from work (still taking the bus there) and a diet change.
Also, I dont know your school, but mine has an extremely inexpensive gym membership and loads of fun classes. Could you kickbox or spin once/twice a week with a roommate/classmate? It makes it fun and social, and an incredible workout!
Both of the above made good suggestions. I also want to note, that taking breaks every half hour or so will help your attention span for the rest of the homework, and help you retain more. Studies have shown (I can't quote an exact one now, but I've read a bunch) that over time your attention to detail, and ability to focus start lagging after about 10-15 minutes for most people, so with the breaks during studying or homework, they will help you refocus your attention back to the work at hand.
Also, remember that weight loss is 80% diet (what you eat) and 20% exercise.
So calculate your calorie target on caloriecount and put your activity level to sedentary. Eat that many calories a day and you will lose weight!
When you do have some time to fit in exercise, input it and you will be able to eat a bit more :)
Exercise is highly recommend for a lot of things, but you can lose weight without it. Set a goal of changing your diet first, and then when school slows down add in the exercise! Small steps lead to success, you can do it!