If a kid was able to do this by saving his allowance, then it can't possibly be very much compared to resources of a school or its surrounding tax base. Give the kids their food.
It's a news article from 2019 for Ryan Kirkpatrick at Westpark Elementary School (a public school in Irvine California). He paid off the debt for only his 3rd grade class (not the whole school, not the whole 3rd grade, just his class). It was a total of $74.50. However, the school still serves hot lunches to kids with a negative balance on their account.
Apparently he paid it anonymously. (???)
Just stating facts above, but yeah, my opinion is that all public school lunches should be free.