Just throwing some ideas out there:
Weight gainers, their cheap, just put them in a aluminum water bottle or something so you don't get odd stares. If your school sells whole milk, that's always good. Brownies, cookies (hell, for varieties sake). Bagel with peanut butter. Pop Tarts, basically any sort of junk food or desert cake (just try to avoid hydrogenated vegetable oil). Putting oils/cream/honey into your protein shakes. Uh, cheese/cheesecake (just throwwing that out there, dunno how'd you'd get it to school though). Fastfood, idk about the quality of cafeteria food but they should have some calorie dense options if my memory serves me correctly. There are also a couple of recipes for protien bars you can find online- normally they add penut butter and oatmeal, plus you can save money and that's always a plus. Soda? (these all depend on how your body processes certain sorts of nutrition) Besides for that, don't worry too much, just make sure you get a good breakfast, and a decent lunch and you'll be good until you get home for your after school snack.