I too gained a lot of weight whilst in Nurses School. I lived in the hospital I trained at... which meant no exercise getting to/ from work. Because of the shift times and the proximity of the hospital canteen usually headed straight for the fast food section.
Don't worry too much about it... I learned that I needed to allot a portion of the day to work out. Really it comes down to a simple equation.... I eat less calories than I burn off during exercise... I don't starve myself, choose foods wisely - and stay away from the hospital canteen. Oh yeah... and I use the stairs rather than the elevators whenever possible.

... now if only I could get rid of the cigarettes...