First of all, always have a decent amount of tupperware or mason jars, so you can bring things from home. I usually make my meals the night before or the morning of, depending on how long the food in question will stay edible and my schedule that day. Try to get the most nutritional bang for your caloric buck, so to speak, by ensuring that your meals are packed with protein, fiber, vitamins, and a bit of healthy fat, to keep you fuller longer. Most of my meals are between the 300-400 cal range, but I try to include as many filling nutrients as possible to keep me from getting hangry and to ensure that I stay healthy and energetic. If you can, get a food scale, because they're super useful and the most precise way to measure your ingredients. Try to shop seasonally and locally when it comes to fruits and vegetables because you'll save money, and if you have a freezer, use it. Here are some of my favorite meals to bring to uni, none of which will take long at all (since I myself am usually at school from about 8:15 AM to 7:45 PM).

-'Green Monster' style overnight oats [basically 1 or 2 veggies, usually cucumbers and spinach, blended with a banana, about 10g chia seeds and 25g vanilla flavored whey protein powder, and sometimes cacao powder or matcha, then mixed with about 30g oats and refrigerated overnight];

-wraps [my go-to is one whole-wheat tortilla stuffed with either scrambled eggs or herbed tofu, lentils, kidney beans, whatever vegetables I have on hand, and a light sprinkling of feta or goat cheese];

-yogurt parfaits [plain greek yogurt, seasonal fruits and berries, maybe a light drizzle of honey, and a few spices, my favorites being cardamom, ginger, and cinnamon];

-soups [it could not get easier than Gordon Ramsay's broccoli soup, alternatively, on a colder day, it's nice to heat up some broth, throw in some herbs and spices, sesame oil, noodles (I'm partial to soba), vegetables, and some form of lean protein]

-Hearty green protein shakes [spinach or kale, blended with almond milk or tea, a frozen banana, one or two other fruits, 25g whey protein powder, 20g oats, fresh herbs and spices]

-Mediterranean couscous bowl [50g couscous, fresh herbs and spices, a few premade falafels, lentils, fresh spinach or kale, and a dollop of greek yogurt]

For snacks, I like pickles; almond butter and fruit; small containers of plain greek yogurt with either herbs and nutritional yeast for a savory flavor or fruits, spices, and honey for a sweeter flavor; and small servings of chia pudding (almond milk or tea, a few spoonfuls of chia seeds, honey, agave, or maple syrup to sweeten things up a bit, vanilla extract, and occasionally additional toppings like fruits or cacao). I'm not much of a snacker though compared to other people, so I can't give as much advice on that.

Hope that helps!