When Cindy and I were kids, we used to bring our school lunch a lot. We definitely brought lunch more days than we bought lunch every week.
Our Dad created a simple system that kept track of how may times we would buy lunch each month. The daily cost for lunch was $1; so we had an envelope filled with $20 in ones, and the numbers 1-20 were written on the outside of the envelope. On the days we would buy lunch, we wrote our name by the number until all the numbers were filled.
Nowadays I try to only bring my lunch, and limit my outside food purchases to once a week. I don’t remember what happened to those extra dollars, if there were any, at month’s end. Perhaps it went back into the pot. If Cindy and I were allowed to keep our remaining lunch money, I imagine we would have used it to buy candy.