In most states funding is related to attendance somehow, I think the details vary. At one time I think there was a thought that districts would take truancy and absences more seriously instead of ignore it if there was an impact to the bottom line.
There are also a multitude of programs throughout k-12, particularly in high school, where the student spends part of their day outside of the district. I think this just helps ensure that the state isn't paying twice for the same student when the external program is also state funded. (It also helps to remove any weird financial incentives to get the kids out of the building)
In my area I think this is where the vast majority of the district income variance comes from, and it is known about and largely planned for before the year even starts.
