Pocket tacos...small bags of fritos, taco meat, toppings. They dump the meat and toppings in the bag and eat out of it (no paperware!) If you can't get the fritos for a quarter, buy the store brand tortilla chips instead...if you shop at a warehouse store you might be able to get a huge box fairly cheaply. Instead of shredded cheese buy an industrial sized can of cheese sauce...much cheaper.
Look for the industrial sized cans of pudding or fruit. Fancy it up with some sprinkles, coconut, or marshmallows. One big can will cost maybe $4.
OR...a potato bar. You can buy a bag of potatoes for about $2.50. Add crumbled bacon, cottage cheese, cheddar cheese, sour cream, and scallions and you have a meal. I sometime do taco potatoes...taco meat, sour cream, cheese sauce, and salsa. Either way is pretty inexpensive.
I know a lot of teens have a more sophisticated palate than that, but, if you're looking for cheap, that's the way I'd go!