A growing question in Memphis and across Tennessee has been why local school improvement efforts seem to be outperforming the state’s 5-year-old flagship initiative.

Now, researchers charged with studying that initiative have a hypothesis: Schools in the Achievement School District have struggled to hold on to their highest-rated teachers.

For their latest report, released on Tuesday, researchers at the Tennessee Education Research Alliance at Vanderbilt’s Peabody College partnered with the University of Kentucky to examine the extent to which the ASD and local turnaround initiatives called innovation zones, or “iZones,” have been able to recruit and retain teachers with top ratings.

They found that ASD teachers left their jobs far more frequently than teachers in iZone schools in Memphis, Nashville and Chattanooga.

That wasn’t a surprise the first year a school was in the ASD, given the requirement that teachers in turnaround schools must reapply for their jobs.

But even in following years, fully half of the ASD’s teachers left its schools each year. Among iZone schools, the corresponding rates were 40 percent and 23 percent, respectively.

In both initiatives, lower-rated teachers were replaced by better ones. Researchers found this to be more pronounced in iZone schools where, on Tennessee’s 5-point scale, incoming teachers scored an average of more than a half point higher than those moving to other schools or leaving the profession. In the ASD, incoming teachers averaged just over a third of a point  higher than outgoing teachers.

“The story seems to be one of general success in getting effective teachers in the door of these turnaround schools, and the iZone schools are also managing to keep and improve them,” said Vanderbilt’s Gary Henry, who co-authored the report.

Henry said disruption is a key part of school turnaround work, and that it might be necessary to lose some bad teachers before a school can thrive. But just as necessary is improving teachers already at a school — and that takes time.

“The iZone hired good teachers, kept good teachers, and their teachers improved,” he said.

Both iZones and the ASD had more difficulty recruiting good teachers for the schools they absorbed in the 2014-2015 school year. Henry said it’s not clear why that happened.

It could be because both the ASD and the Memphis iZone, the largest of the three, added high schools, and it’s typically harder to get effective high school teachers to switch schools. Or, it could be that Memphis, where nearly all of the ASD schools are located, needs more good teachers in general.

“Memphis might be reaching a ceiling on the number of effective teachers willing to move into priority schools,” he said of schools that are academically in the state’s bottom 5 percent. “They’re going to have to expand their pool in order to attract the type of talent needed to transform the lowest-performing schools.”

The researchers note that the iZone gains might not last. The one in Memphis has used teacher pay incentives to lure high-quality teachers to its schools, relying at least in part on philanthropic funds. Without those funds, it’s not clear if the iZone could be expanded or sustained.

“It’s terrific when philanthropies are able to support mechanisms proven to work,” he said, “but in the long run, it’s uncertain whether Memphis will be able to maintain these gains.”

ASD Superintendent Malika Anderson said she is heartened that more effective teachers have moved to working in historically low-performing schools. She attributed the ASD’s initial recruiting challenges to being “a big unknown,” but expressed optimism about the future.

“As we increase recruitment and retention of effective teachers in our schools, the ASD’s growing priority is to champion the efforts of local districts, community partners and the Department of Education to strengthen the pipeline and critical supports for effective teachers in all schools,” Anderson said in a statement.

This report follows a high-profile 2015 study that showed schools in Tennessee’s iZones had positive effects on student learning, while the ASD’s effects were statistically insignificant.  Henry said Vanderbilt researchers hope to examine in the future how school quality was impacted at the schools left by highly rated teachers to go to the iZone or the ASD.

You can read the full report here.