No such explanation is needed; in the lack of powerful motivating forces for education (or anything else) to be good, it will be mediocre. PG makes the point that school is bad, not because it was deliberately designed to be, but because the design spec was to warehouse otherwise potentially disruptive young people, not particularly to educate them: http://www.paulgraham.com/nerds.html

"""And as for the schools, they were just holding pens within this fake world. Officially the purpose of schools is to teach kids. In fact their primary purpose is to keep kids locked up in one place for a big chunk of the day so adults can get things done. And I have no problem with this: in a specialized industrial society, it would be a disaster to have kids running around loose."""