When the Tennessee Department of Education issued its list of lowest-performing schools in 2012, Memphis was a glaring hotspot, the home of 69 “priority” schools scoring in the state’s bottom 5 percent, paving the way for intervention at multiple levels.

This year, all but 15 of those schools have either been shuttered or are under the oversight of intense turnaround initiatives implemented by Shelby County Schools or the state’s Achievement School District (ASD).

And next school year, that number will dwindle to eight following decisions this month on more interventions.

Shelby County Schools announced Wednesday that its Innovation Zone will absorb three additional high schools — Douglass, Mitchell and Westwood — bringing to 21 the number of priority schools that will be part of the local iZone in 2016-17.

“The current academic status of these schools illustrates the fact that we have not been effective enough in supporting students,” said Shelby County Superintendent Dorsey Hopson in a statement. “We have a responsibility to do things differently in order to improve achievement at a more aggressive pace.”

Historically, all three high schools have struggled academically. While their state achievement test scores were on the rise last year, most students failed their end-of-course exams for 2014-15. They also scored four or more points below the state’s ACT average of 19 — and six points or more below a score of 21, which is considered college-ready.

The latest priority schools being taken over by the ASD also have had chronic challenges. Last week, the state-run district announced it would add four other Shelby County schools to the 27 it’s already overseeing in Memphis.

“To ensure that these students and schools are getting what they need, it couldn’t be a solo act by the ASD, it couldn’t be a solo act by the iZone. This is what we’re working on together,” said incoming ASD superintendent Malika Anderson.

The ASD has been the primary driver of massive turnaround work in Memphis, either through schools under its own oversight or through the takeover process that has cost Shelby County Schools enrollment and funding and prodded the local district to more aggressive action.

But while ASD officials often paint a picture of cooperation and collaboration, local district leaders have been more vocal in venting their frustration with each ASD takeover. Last week, their concerns were validated independently when researchers at Vanderbilt University’s Peabody College released a report calling the state-run district’s academic impacts marginal thus far and suggesting that priority schools in Memphis would be better off in the iZone.

That report prompted Shelby County’s school board to unanimously pass a resolution this week placing a moratorium on the ASD taking over more schools “until they show consistent progress in improving student academic achievement.”

The iZone has been one of Shelby County Schools’ most successful initiatives since being launched in 2012 under a new state law giving flexibilities and federal money to traditional public school districts to improve chronically underperforming schools. In four years, 11 of its schools have boasted double-digit test score gains, and seven have moved off the state’s priority list. Additionally, the program and its leader, Sharon Griffin, have garnered numerous accolades, including praise from U.S. education chief Arne Duncan during a visit to Memphis in October.

Unlike with the ASD model, which depends primarily on nonprofit charter networks to manage schools, iZone schools remain in the local district. However, like charters, its leaders are given flexibility to hire and fire staff, overhaul their curriculums, give their teachers bonuses, and add time to the school day, among other things.

The iZone model is expensive to operate, mainly because of costs related to adding an hour onto every school day — the equivalent of 23 more school days in a year.

In September, the district received a three-year, $10 million philanthropic grant for the iZone from Teacher Town Memphis, a coalition of national and local funders who wish to remain anonymous. School leaders said that the grant would ensure the iZone’s expansion next year.