Ever since I have heard about protein increasing performance, all I have been eating during the school day is protein, protein, protein!
Examples of what I eat (and some of this may sound weird, but I like them).
Air popped popcorn sprinkled with protein powder (make the popcorn taste better and you don't get weird looks about having protein powder because it's on the popcorn)
Tuna, chicken, cheese, ham, those long life milk things (like plain, not big m's), nuts, chickpeas, lentils, baked beans and plain beans, leftovers like bolognese pasta sauce, curry. Homemade muesli bars (but not store bought because they're full of sugar and will make you get rumoured year 12 belly in no time!)