Really man, what you eat isn't going to make the "biggest" difference. Unless you're pretty open and you don't give a shit if you pull out a cold potatoe and cooked chicken breast, I'd just eat whatever, a sandwich or something, and just try to get enough in to fuel you for your workouts. I dunno how old you are but you can always just eat clean after school..
I used to pack chicken breast and a cold 8 oz part of a cooked sweet potatoe. Trust me, I didn't get big from eating that shit once a day by myself in the cafeteria, just eat whatever, not worth doing what I did.