There is healthy, and there is improvement. I'm in a similar situation, and generally dislike spending significant amounts of time cooking. I try to avoid garbage junk food or anything that's pre-made. So what I'm listing isn't going to be ideal, but it will be relatively cheap and fast, hopefully without being outright unhealthy. Some easy things you can do:
Burgers! Get access to a bbq if possible or use a george foreman (< $20). Grab lots of fresh lettuce and tomatoes and pickles. The foreman is good for any meat, as long as its not too thick. Burgers take 5-10 minutes tops. You can buy cheap burger patties by the bag for pretty cheap.
Sammiches. Tuna fish can be alright, but you have to mix the tuna and mayo and putz around with that. You can always rock the PB&J or PB&honey. Grab some pre sliced meat and cheese that comes in the re-sealable bags. Turkey is probably healthiest. Nice thing about sammiches is it overlaps nicely with your burger supplies, and they're modular. Meaning you can do a simple condiments+meat+cheese, or go for the whole shebang and add lettuce, tomato, pickles, + any other veggies you can stand.
Macaroni and cheese! hell yeah. You can get generic kinds for < .50 cents, just need some milk + cheap margarine or butter. Takes 12-15 minutes, but a good chunk of that is boiling the water, so you can do other stuff while you're making it.
Eggs in any shape or form are good. Scrambled is probably the fastest. I ate so many eggs in the past couple of years I'm kind of sick of them now. They're kind of cheap, but consider that you only get 2-3 decent meals out of a dozen.
Granola! Tasty and probably healthier than regular ceral, though I'm partial to a knock off brand of frosted mini wheats myself (can be bought in bulk, in a bag, about half the price of kellogs). If you're feeling adventurous, you can grab some yogurt or slice up some fresh fruit stick it in the granola. Pretty tasty.
Soup! Heat and serve. I usually go with Progresso lentils, then toast a bagel + cream cheese and use it as a sop/spoon with the lentils. usually everyone looks at me weird, but i likes it.
French toast! Good with a roommate to split. Get a loaf of bread (or more!), a buncha eggs, and some milk. You'll probably need 8+ eggs to do an entire loaf. Whip up the eggs in a bowl large enough to dip the bread slices in, then add however much milk you feel like and spice with Cinnamon if desired. You can cook these in a frying pan, but its much better if you can score a cheap gridle. make way more than you can eat, and then wrap the rest and put it in the fridge. last a long time and you can nuke it and add syrup pretty quickly.
Sometimes you can score some cheap steak. I like to keep an eye on petite sirloin, which can go down to 2.99 a lb. Assuming you can get some, also pick up some powdered potatoes, which are actually really tasty. Also grab some frozen veggies and you can have yourself a balanced meal. you can broil the steak in an oven, or slice it up and cook it on a foreman.
Grab some olive oil and fry up some chicken. pretty easy and quick, and flexible. get some pasta roni thingies and mix, they have a bunch of different kinds. sometimes the pasta by itself is enough of a meal.
You should try and have some fresh fruit around. Bananas always seem to be cheap. Other fruits it depends on the season and stuff. Grab a fruit on the way to class in the morning. Personally my staples for fruits and veggies are bananas and carrots. Good work ditching the pop, and try to limit yourself to 1 bag of chips and crackers a week. I usually get the baby carrots, so they're probably minimally nutritional, but hey, what can you do?
How's your kitchen tool supply? You should definitely get:
A strainer. For pasta, which you should try and make a lot of since it's easy and generally can be combined with lots of different sauces for variety.
A pot. Pasta, veggies, soup, etc.
A frying pan. For eggs an whatever else.
A small george foreman grill. for quick meat cookin and cleanup.
A cheap gridle if you can find one (try walmart). useful for eggs, french toast, pancakes (also easy, just buy the dried powder mix and follow directions).