Thanks for repeating this. I started saying it yesterday. I think there is one official counting day, where the county or whoever does a physical head count. That number gets them their budget money. Our schools send a letter urging parents to make sure their kids are in school that day. If you want to influence the principal, tell them you will keep you kids out on THAT day.
It's not one day, it's every day.
Schools get paid according to the number of students each day. In schools with multiple classes, they choose one period -- I believe it was the 2nd period in mine.