Soup is easy to make in batches; legume-based soups freeze well and others freeze at least tolerably. Measure out servings into (well-sealed) tupperwares and freeze them; chuck one into your backpack come morning. Pasta salad, potato salad, egg salad, or tuna salad can also be made in large amounts - they won't freeze as nicely, so you'd want to put them next to your frozen soup or one of those things that you put in lunchboxes to keep stuff cold.