Remember, we're talking about absenteeism and not truancy. Absenteeism means absent from class. If you are excused and not in class you are still absent. Doesn't matter if it's to help on the farm, see a doc, or attend a ball game. You are still "Absent".
So, we all hear about the importance of sports and extra-curricular activities. Out of town games, science fairs, music, FFA meets and a bunch more will inevitably lead to absences. It's a good bet that Friday will have a bunch of excused absences, so there's your 20%, baked in to the statistics. So what to do? Here's an idea; let's move the goalposts. Pretty common, these days to only have four days of classes and everyone gets Friday off. Teachers like it, students seem to. Households where two parents work aren't wild about it because it's one extra day where they need to arrange child care. 
Coming soon, to a district near you, if it's not there already.