The District has higher-than-desirable teacher turnover, but a report released Thursday finds that the public school system is succeeding in holding onto its best teachers at nearly twice the rate as its lowest performers.

Still, far too many excellent teachers are leaving the school system each year, according to TNTP, a nonprofit teacher recruitment organization that conducted the study.

“On the one hand, they’ve gone where no district has ever gone before: They’re retaining far more of their best teachers than their worst,” TNTP President Timothy Daly said. “On the other hand, they’re still losing too many of their best teachers for reasons they could address.”

The study, which included surveys of nearly one-quarter of the school system’s teachers, suggests that some teachers leave because of poor working conditions, weak school leadership and school cultures that lack mutual trust and respect.

Many others could have been persuaded to stay, the study found, but no one made an effort to keep them. More than two-thirds of District principals did not list talent retention as one of their top five priorities.

Former D.C. schools chancellor Michelle A. Rhee founded TNTP 15 years ago, and Chancellor Kaya Henderson served as vice president. The organization has a contract to provide teacher-recruitment services to D.C. public schools, but no money was spent on this research project.

TNTP studied four other urban school systems this year and found little difference in the retention of high-performing and low-performing teachers, as judged by students’ progress on standardized tests.

The District treats those two groups of teachers differently, and that sets the school system apart, TNTP found.

At the end of the 2010-11 school year, District officials retained 88 percent of teachers rated “highly effective” under IMPACT, the school system’s rigorous and controversial teacher evaluation system.

It retained only 45 percent of teachers rated ineffective or minimally effective. Many of the low performers who left the system were forced to do so; the school system has fired nearly 400 teachers for poor performance since 2009.

Overall, more than one in five teachers left at the end of the 2010-11 school year, higher than four other urban school districts TNTP studied, and more than three times the turnover rate in suburban Montgomery County.

“We’re very proud of the fact that we’re differentially retaining our teachers,” said Jason Kamras, the school system’s chief of human capital. “That is not a small thing, and that has real benefits for kids. But we certainly acknowledge that we have room to grow.”

Kamras said school system leaders have made changes aimed at encouraging principals to be more thoughtful about retaining good teachers. Starting this year, for example, retention rates factor into principals’ job evaluations.

The TNTP report confirmed a fact long debated among District educators and politicians: Highly effective teachers are less likely to teach in schools with large numbers of poor children. That either means that few of the city’s best teachers are working in needy schools, or — as many teachers contend — the IMPACT evaluation system is unfair to teachers working in the most difficult environments.

“I think, at the end of the day, we’ll find out that it may be a little bit of both,” Kamras said.

Meanwhile, the school system isn’t going to force highly effective teachers to transfer into needy schools. Instead, it has revamped the bonus pay system this year so that successful teachers in high-poverty schools can earn thousands more in merit pay than peers in low-poverty schools.

Mark Simon, a D.C. parent and activist who has tried to raise an alarm about high teacher turnover in the city, said the report’s recommendations to improve school culture fail to account for the possibility that IMPACT could be a key factor driving teachers away.

Highly effective teachers leaving the District ranked the evaluation system as fourth of 20 reasons for departing, according to the TNTP report. The school system made several changes to IMPACT this year to reduce teacher anxiety about it.

Toni Conklin, a highly effective teacher who retired from Bancroft Elementary School in spring, said the evaluation system has contributed to a “tremendous amount of stress and pressure” teachers feel on the job.

She decided to leave a year earlier than she had originally planned because of frustrations with leadership at her school. When she submitted her letter of resignation, she said, no one asked her why she was leaving or if there was any possibility that she would reconsider.

Bancroft Principal Zakiya Reid said she would have been happy to keep Conklin on staff but wanted to be “supportive” of the teacher’s decision.

“Given Ms. Conklin’s tireless dedication and service to our school community for almost 20 years,” Reid wrote in an e-mail, “I felt that I could only respect and honor Ms. Conklin’s decision to retire.”