Dang sounds like such a pain, I remember in elementary just going down the lunch line and at the end would be a old lady on a stool and little cash box you paid, if not she would write you up and send you home with a note that you owed money haha. In high school we had the free lunch line in a building and the outside had some containers that sold food like chicken sandwiches or cup noodles for cash.

Could you use cash?