I assume you mean food to TAKE to school and not to make for breakfast. So I'll also assume you mean things that don't need to be heated.
Sandwiches are always great -- just try mixing them up. Get tortillas or wraps to use instead of bread.
I also like to get a plastic container and fill it with "finger foods" -- rolled up lunch meat, boiled eggs, veggies, fruit, cheese, whatever i want. I just put in the container. Kind of like a non-Asian bento box.
How about pasta salad? You can cook that a few days in advance and as long as you keep it in the fridge, you can get out a serving to take with you. Non-mayo and non-cream dressings are better--like italian. I like to use tri-color rotini, diced chicken and then whatever veggies I like in my salad (I use peas, tomato, carrot, cucumber, and green olives usually) -- then I toss with italian dressing. YUM!