The emphasis on better teachers—through training, selection, or dismissal—comes from the very consistent finding that improving faculty is one of the best, most reliable ways to improve schools. If the person standing at the front of the classroom has raised the test scores of students he's taught before, he's likely to do so again.
But how do you get good teachers in the classroom? Unfortunately, it turns out that most evidence points toward great instructors being born, not made. National board certification may help a bit, a master's degree in education not at all. It's also difficult to pick out the best teachers based on a résumé or even a sample lesson. It takes a year or so before evaluators (and even teachers themselves) know who is really good at getting kids to learn, and few qualifications are at all correlated with teaching ability. Candidates with degrees from prestigious colleges—the type where Teach for America does much of its recruiting—do a bit better, but not much.
The only option left on the table is getting rid of bad teachers once they're already teaching—perhaps by firing low-performing instructors after a probationary period of a couple of years. How many teachers would school reformers have to fire in order to get American schools performing at their best? That's the question researchers Doug Staiger and Jonah Rockoff set out to answer in a study they presented at the Columbia conference. ...
When they ran the numbers, the answer their computer spat out had them reviewing their work looking for programming errors. The optimal rate of firing produced by the simulation simply seemed too high: Maximizing teacher performance required that 80 percent of new teachers be fired after two years' probation.
After checking and rechecking their analyses, Staiger and Rockoff came to understand why a thick stack of pink slips are needed to improve schools. There are enormous costs to having mediocre teachers burdening the school system, and once they get their union cards, we're stuck with them for decades. The benefits of keeping only the superstars is enormous, such that it's better to risk accidentally losing some of the good ones than to have deadwood sticking around forever.
--Ray Fisman, Slate, on the benefit of extreme up-or-out in schools