I imagine they can treat absences the same as they currently are. Teachers are told in advance when a child/children will be absent and can plan accordingly. I remember being given notes and the work I was going to miss in advance and needing to catchup. If teachers see that a lot of kids will be absent, they can plan alternate activities. Or maybe for a week a month there will be week long camp at school, which will be easier for students to be absent since they won't miss class.
This is all theory I have not developed out. I would like to hear someone's thoughts on changes to how the US school system runs.