I work and go to grad school at night, so I feel you on those long days. I have a microwave in the office, so I make and freeze lots of individual portions of chili and stews that i can grab and reheat later. I also make lots of salads (not big leafy ones- grains, beans, meats). Pre-portioning is your best friend. If you don't have a fridge or a way to reheat, basic sandwiches work well- cold cuts, avocado or cheese, pile on other veggies.
In a pinch- Subway is my best friend. There's one near school so when I've forgotten to pack dinner I go there- 6", low fat, every veggie they have.