" Rather, they would insist that it's a more organic experience of learning through teaching over time and experimenting with the medium"

Yeah, my anecdotal understanding of the profession acquired through similar channels would corroborate this. The sad thing is that in strict licensing situation the great teachers who lack proper licensing are locked away (e.g. when licensing requirements change).

I suppose university degrees work mostly as gateways to the careers that require them - i.e. it is thought the candidates who graduate are better qualified for the job than those that did not. I have no idea if the people filtered out would actually be the ones who would perform worst on the job.

"To me the ultimate effectiveness measure is if parents choose to send their kids to that school or students sign up for that class voluntarily."

I don't think parent perceptions kick in as a significant metric of quality until the differences in school quality have reached pathological levels.

And in the first grades kids are not really supposed to sign in to classes by themselves.