This may be true, but it's the result of how the system is set up and, in particular, the interaction between:
a) politicians
b) teachers and teachers unions
c) school district leaders
d) school district administration staff
e) students
In most systems, group (e) is the one with the least power. Their parents may be able to vote for school board members, but these elections are infrequent, and the way to get elected is not by being an advocate for students, but by aligning with a particular political party, or getting the support of teachers unions.
Parents cannot easily opt out of that system. In Alabama, per pupil spending is ~$10k/year. This distorts the market: a parochial school that costs $10k/year to run cannot compete with a public school that costs $10k/year to run, even if the former better. Because in one case the marginal cost to the parent is $0.
It comes back to what I said at the start of our conversation: The market for teachers, at least in the US, is not a 'laissez-faire market'.
I've said it already - it's not the best students that go to work in teaching. You can fire a low performer, but chances are that the replacement is just as bad. It's a supply issue, a training issue and a culture issue - the anti-intellectualism in large swathes of the population really doesn't help matters.