Researchers have been predicting a precarious outlook in American classrooms for years on account of a looming teacher shortage.

Just as a severe drought has major downstream impacts on numerous aspects of life, so too has it been argued that a shortage of teacher candidates would wreak havoc on student learning in America. The shifting demographics of the baby boom era, comparatively increasing salaries in other professions, and a general decline in the quality of workplace conditions for teachers, it's been argued, would lead to a shortage that in turn would impact student achievement as principals and school districts struggle to find any warm body to stand in front of a classroom.

So much for that doomsday scenario: the economic downturn has led to not a shortage of teacher candidates, but a surplus.

What will the impact of this surplus be? One might be tempted to conclude that if a teacher shortage would lead to lower teacher quality and reduced student outcomes then a surplus of available candidates might logically lead to increased teacher quality and student outcomes, as principals and districts suddenly have their pick of the litter in terms of which teachers to hire. As one district official pointed out, "It is a tougher job market, and you get applicants that you might not normally have because of the economy."

And in general, that labor market logic should work. After all, if you're the boss of a company and you need to hire an employee, much better to have 20 people to interview and pick from than 5. Even if you don't have any job openings, your new-found ability to shop around might enable to you to replace a previously under-par employee with a better one since so many candidates are newly on the market.

But in U.S. public education, I'm afraid that the shift from a teacher shortage to a teacher glut operates only in a one-way ratchet for students: even when schools in theory have more candidates to hire from, it's highly likely that there will be little measurable improvement in the quality of a child's teachers in the aggregate.

The main reason is that it is so hard to replace ineffective teachers. To be sure, schools that have new openings to fill will likely have greater success these days in finding a good teacher than five or ten years ago, as the AP article discusses. But administrators in schools with bad teachers who might want to replace those teachers are out of luck to the extent that removing bad teachers, even for cause, is a nearly impossible enterprise due to teacher tenure rules.

A Kansas deputy superintendent, John Black, put it best, "Now we have these great applicants wanting to teach, and we don't have jobs to offer them." With due respect to Mr. Black, I'd point out that they do have jobs to offer them, but those jobs just happen to be filled currently by low-performing educators who are all but impossible to remove.