My assumption is if they have 500 kids they make 500 lunches, but some kids will be sick so teachers are allowed to take a lunch if there are some left over. But then some teachers were missing out so probably some rivalry to get in first and eventually this evolved to getting in before all the kids have eaten.

Also maybe they assume say 5% of kids will be sick each day, which would mean the number of lunches left over may vary a lot depending on actual attendence.