If we assume that it's impossible to measure performance (in any job, but also when applied to teaching), then market salary is minimum wage. We know we have u

If we assume that it's impossible to measure performance (in any job, but also when applied to teaching), then market salary is minimum wage. We know we have unemployment i.e. there are people available that would take the job. If you can't tell if a person A does a given task, in this case teaching, better than a random person from unemployed pool, the rational thing to do is to hire a random unemployment person for minimum wage. That, of course, would be absurd: we all know that there are differences in performance and they can be measured and they are being measured. As everything fuzzy in life, they are not always being measured perfectly, but that doesn't mean we should throw the baby with the bathwater by declaring that because we can show that sometimes the measurements aren't fair, they can never be fair. That's a fallacy and even article in question is based on assumption that you in fact measure the quality of teachers.