1. You can't send the kids home because both parents are probably working. It's more likely the kid becomes lost forever and it becomes a government problem again once that kid is now living an alternative lifestyle that consumes government resources in other ways (crime, homelessness, or health).
2. Because of the realities of (1) everyone else is now forced to asked why the kid is no longer in school and becomes a failure of the teacher that the kid is not in school. The teacher must now deal with disgruntled parents and administrators for not enduring the emotional abuse and disruption a neglected child causes while also dealing with 20 other students.
I think a lot of problems with child education and just be traced back to economic realities of many parents today. Even if you are relatively well off having your kid kicked out of school isn't financially feasible for a lot of families. Even a disruptive child might be the result of absent parents who must be at work to even afford school lunch.