On Wednesday, I stirred a bit of a hornet’s nest when I wrote, “I think that...many people teaching today probably shouldn’t be.” Given the charged response from readers demanding that I justify this assertion, I’ll say a few more words.

First, it strikes me as a banal, unremarkable statement, one that I’ve uttered regarding attorneys, professors, journalists, salesmen, federal bureaucrats, think tankers, and district administrators. In this context it wasn’t intended as an attack on educators, which is what made the heated response so noteworthy. People vary in talent, energy, and performance, and this means there are poor performers everywhere--even in fields with relatively stringent selection or hiring requirements.

Second, education hires a lot of educators. We’ve 3.4 million teachers in the U.S., which represents more than ten percent of the college educated workforce. That’s twice the number of lawyers and doctors, combined. The more people you need, the more challenging it is to ensure quality. In 2005, The New Teacher Project (TNTP) reported that, “Urban schools are forced to hire large numbers of teachers they do not want.” It’s no surprise that supes struggling with class size mandates, from Florida to California, have told me they’ve sometimes had to hire lousy candidates just to fill classrooms.

Third, the challenge is aggravated by weak quality control. As I wrote Wednesday, “Teacher education programs and school districts generally do a mediocre job of preparing educators and a pretty awful job of screening out lousy educators.” Several years ago, University of Texas professor David Leal reported that teacher preparation programs actively screen out about two percent of aspiring teachers (including during candidates’ student teaching). In its 2009 The Widget Effect report, TNTP reported that districts consistently judge 99 percent of their teachers to be satisfactory, suggesting (in TNTP’s estimation) that district performance evaluation is broken.

Fourth, teachers themselves say that they teach alongside colleagues who shouldn’t be in classrooms. Public Agenda has reported that 78 percent of teachers say there are at least a few teachers in their school who “fail to do a good job and are simply going through the motions.” In a Public Agenda focus group, one New Jersey union representative confessed, “I’ve gone in and defended teachers who shouldn’t even be pumping gas.”

My take is also informed by a half-decade supervising student teachers and research and school observations in a lot of districts. But let’s keep it simple. If 75 percent of the nation’s 100,000 schools have at least a couple teachers who shouldn’t be teaching, that means teachers themselves are reporting that 150,000 or more of their colleagues probably shouldn’t be teaching. In my book, that’s “many.”