Comment by Gormo

Learning how to co-exist with different sorts of people is definitely a valuable experience.

Trying to do that in an completely artificial institution that arbitrarily divides people into age cohorts in a way that resembles no organic social pattern and forces all social interaction to conform to bureaucratic rules is not just not a value experience, but in fact actively inhibits the above goal.

The kinds of social skills and expectations kids develop in a school environment often need to be unlearned entirely in order to function effectively in a complex and dynamic society.