In the past few weeks, two major reports on teacher turnover and retention have been released. One was rolled out with extensive media coverage, and has been the subject of much discussion among policymakers and education commentators. The other was written by me, along with Teachers College doctoral student Clare Buckley.

The first report, “The Irreplaceables: Understanding the Real Retention Crisis in America’s Urban Schools,” was prepared by TNTP, an organization formerly known as The New Teacher Project that prepares and provides support for teachers in urban districts, and that advocates for changes in teacher policy. The second, “Thoughts of Leaving: An Exploration of Why New York City Middle School Teachers Consider Leaving Their Classrooms,” was released by the Research Alliance for New York City Schools (RANYCS), a nonprofit research group based at New York University. (RANYCS published a report by Will Marinell in February 2011 that examined detailed patterns of teacher turnover in New York City middle schools apparent through the district’s human-resources office.)

There are some important similarities between the two new reports. Both surveyed teachers in large urban districts about their plans to stay in their current schools or to depart either for other schools, other districts or other careers. Both also sought to understand the features of teachers’ work on the job that were influential in their plans to stay or leave. The study of New York City relied on a large, anonymous sample of middle-school teachers: roughly 80 percent of the full-time teachers in 125 middle schools across the city. In contrast, the TNTP study surveyed smaller numbers of teachers in four urban districts (one of which appears to be New York City), and the surveys were not anonymous, because TNTP wanted to link teachers’ survey responses to what the authors viewed as measures of teachers’ performance, such as value-added scores or summary teacher evaluations.

The headlines from the two studies aren’t that different: In any given school, many teachers think about leaving, and it’s not easy to predict why some teachers are more poised to move than others. The NYC study suggested that the rhythms of teachers’ lives matter, including their pathways into teaching and the positioning of teaching in a life with adult family responsibilities. The teachers prepared through alternate routes such as the NYC Teaching Fellows and Teach For America—26 percent of those surveyed—were more likely to consider leaving their classrooms and schools, even when other teacher characteristics were taken into account. And teachers who were separated, widowed or divorced, and those with responsibilities for raising children, were less likely to think about leaving, perhaps because of the financial risks. Commuting, too, takes a toll, with teachers who commute an hour or more each way to their jobs more likely than those with shorter commutes to think of leaving their current schools—but not more likely to think about leaving teaching altogether.

But regardless of teachers’ biographies, administrative leadership and support—and student behavior and discipline—matter a great deal. Teachers are more likely to consider leaving their classrooms if they believe they aren’t getting adequate support from their principals, and if they believe the school doesn’t function well as an organization. Good leadership is not randomly distributed among schools; on average, NYC teachers report less satisfaction with the leadership in schools serving high concentrations of low-achieving, high-need students.

The key divergence between the two studies is that the TNTP report sought to identify high-performing teachers—whom the authors labeled “irreplaceables”—and low-performers. These groups, the TNTP authors believe, are stable; a teacher identified as a high-performer early in his or her career is likely to stay that way, and low-performers, although they may work just as hard, unfortunately rarely get better. Rather than try to provide extensive support to struggling teachers early in their careers, TNTP argues, it’s more efficient to invest in retaining the “irreplaceables,” and to counsel out—or move more aggressively to push out—low-performers who may well be replaced by teachers who will be “better.” To date, the authors suggest, principals have not been this strategic, leaving who stays and who leaves pretty much up to chance.

I’m less sanguine than the TNTP authors about the ability to easily identify those teachers who are “irreplaceable” and those who are—what? Expendable? Disposable? Unsalvageable? Superfluous? The terms are so jarring that it’s hard to know how a principal might treat such a teacher with compassion and respect. Given what we know about the instability from year to year in teachers’ value-added scores as well as the learning curve of novice professionals, a reliance on a rigid classification of teachers into these two boxes seems unrealistic.

I don’t doubt that there are some individuals who are natural-born teachers, just as Michael Phelps has shown himself to be a natural-born swimmer, and perhaps their talents are revealed on Day One. But there are thousands and thousands of children and youth around the world who are competitive swimmers, and none of them is Michael Phelps. For these children and youth, as for most teachers—and there are approximately 3.5 million full-time K-12 teachers in the United States—technique and practice can yield great improvements in performance. This is perhaps even more true in teaching than in swimming, as there are many goals to which teachers must attend simultaneously, rather than just swimming fast to touch the wall as soon as possible.

Principals must, it seems, strike a delicate balance, seeking to cultivate a professional community of successful teachers through a mix of selection, “de-selection” and professional development. But even in systems that view principals as “mini-CEOs” of their schools, knowledge of teaching practice is distributed throughout the school and district.

It’s true that teacher professional development is often weak and ineffective, and, particularly in the early career, probably requires a more coherent strategy and division of labor than currently exists in most school districts. But that’s not a convincing rationale for giving up on professional development for all teachers in favor of the quick termination of those teachers who don’t hit the ground running.

There’s a reason revolving doors are frequently out of order.

Arne Duncan famously said, “You can’t fire your way to the top.” TNTP apparently disagrees. For once, I agree with Arne—mark the date.