A person with 20 years experience in the school system who has worked at all levels tells me this:
1. Parents start diminishing the importance of school in the first grades, even kindergarten, with too many reasons to pull them out for a day or two. "We wanted to go to Portland".
2. Poverty and homelessness means parents just disappear from districts. Then they come back.
3. Rural parents will remove kids from school to go hunting: "Puttin meat on the table is more important" They will expect the teacher to develop a week of work to replace the class but she says it seldom comes back. Parents will remove kids who are not passing.
4. By high school the pattern is set. Schools call parents and leave a message the kids were not in school. But a parent with little control does not do much to stop this.