For most things, if you make a sandwich, etc in the morning before school and eat it that day, it's not going to go bad by lunch time. I bring Yogurt to work every day and it doesn't sit in the fridge, just a thermal lunch bag, and it's fine when I go for lunch.

Also you can make pasta and sauce, divide it into lunch size tupperware and bingo - you have on the go lunches that you can reheat really quickly!