There's plenty of debate about the need to take ineffective teachers off the payroll, but where does the line get drawn between "effective enough" and ineffective? Looking at patterns of performance, Doug Staiger and Jonah Rockoff estimate that as many as 80 percent of all new teachers should be let go, and with haste.

Even factoring in a flawed teacher evaluation system that all too often means plenty of misses (and some friendly fire), they argue that the benefits to student achievement are too big not to make such a move. In terms of student achievement, the cost of keeping an ineffective teacher is far greater than the cost of accidentally firing a good one. According to Staiger and Rockoff, a 0.1 standard deviation drop in a student's learning in a single year corresponds to a $10,000 to $25,000 decrease in that student's earnings as an adult, much greater than the cost of hiring a new teacher.

Not pulling any punches, their model shows that waiting two or three years to gather more detailed information on teachers having been pegged as low performing will only diminish what otherwise would be blockbuster student results. Still more punch could be gotten if districts improved their evaluation processes, and even more still if they did a better job identifying potential recruits at the point of hire who are most likely to succeed.

Staiger and Rockoff aren't completely hard-nosed. While asserting that the very worst teacher should be fired immediately, they note that waiting an extra year or two to assess borderline teachers would still benefit kids.

Some bold postulations for sure, but we're made a little nervous by the sterile lab conditions. There's an imperative lesson herein, but it shouldn't be wholesale firings, but an end to a national system of teacher preparation that screens too few aspirants and then far too often adds no value. In fact, Staiger and Rockoff suggest that anyone with a college degree be allowed to teach, and that good teachers be identified after entering the classroom. Pre-service teacher education may appear to many to be worthless, but its questionable value should shock us into action, not resignation.