NEW YORK — Mayor Bill de Blasio is canceling one of his signature education initiatives, acknowledging that despite spending $773 million he was unable to turn around many long-struggling public schools in three years after decades of previous interventions had also failed.

The end of the initiative, called Renewal, is a blow to de Blasio, who had hoped that success would bolster his effort to build a national reputation for innovative policies. Urban educators around the country had also looked to Renewal as a model for improving underperforming schools in historically troubled districts, rather than closing them.

Instead, the program has been plagued by bureaucratic confusion and uneven academic results since de Blasio began it in 2014. Though some of the nearly 100 low-performing public schools have shown better results, many have fallen short of the improvements that de Blasio predicted. The Renewal label itself caused parents to seek other options, causing enrollment in some schools to plummet.

The New York Times reported in October that de Blasio was preparing to close Renewal, and that city officials had known some Renewal schools were likely to fail but had left most of them open anyway. As a result, officials essentially kept thousands of children in classrooms where they had little if any chance of thriving.

The question of how to fix broken schools is a great unknown in education, particularly in big city school districts.

While some small cities like Lawrence, Massachusetts, and Camden, New Jersey, have achieved some success with different strategies, no large school system has cracked the code, despite decades of often costly attempts.

As education fads have come and gone, politicians have flipped between school improvement models based on punitive measures like closure and teacher firing and softer approaches that rely on pouring resources into schools.

In a lengthy interview Monday, de Blasio defended the goals of the program, but said its execution was flawed.

“I’m at peace that with the information we had and the structure we had at the time, it was a sensible approach,” he said. But he added, “I would not do it again that way.”

De Blasio said the reorganization of the Department of Education’s behemoth bureaucracy, undertaken last year by his schools chancellor, Richard Carranza, would help smooth out the confusing lines of authority that led to frustration among principals and teachers in Renewal schools.

Though only about 25 percent of the schools in the program have improved enough to require less investment and support on the four-year timeline de Blasio established, the mayor insisted that Renewal still achieved its “core mission.”

“We never said from day one, ‘We guarantee all of them will become paradise,'” he said, referring to the schools in the program. “We knew that there could well be closures and ones that didn’t work.”

“We defined success as whether most of the schools could move substantially in the right direction — and that, we got,” de Blasio added.

Though about a quarter of the schools improved enough to exit the program after three years, another quarter closed, and the remaining 50 will remain open but in a limbo state: They will continue to receive indefinitely much of the same support they did as Renewal schools, but without the Renewal label. The city could still close any of these schools in future years, officials said.

De Blasio said he is confident that those schools are “on a path for success,” even though they are taking more time to change than his administration first hoped.

“We put ourselves on this very aggressive three-year timeline. In retrospect, that was probably an unrealistic timeline in some cases,” he said. De Blasio eventually added a fourth year to the program.

Renewal will not be replaced by an entirely different school improvement program. Instead, the city will direct funding and academic support to schools most in need of help, under a strategy it is calling comprehensive school support, by consulting student achievement data under a new centralized data system known as Edu Stat.

That strategy will technically apply to all schools in the system, though the initial focus will be on schools that state Education Department officials consider low-performing. Schools will not be publicly labeled low-performing, in part to avoid the stigma that the Renewal label gave schools, which city officials believe led to dwindling enrollment as parents searched for other options.

The Renewal schools that did succeed tended to have particularly strong principals, according to a report prepared by city officials about the end of the program, and an emphasis on using student achievement data to identify problems. The high schools in the program, which used a structured data-tracking system, made the most collective progress: The average graduation rate rose to 72 percent last year, up from 52 percent in 2014.

Suspensions also dropped by more than 50 percent across all Renewal schools during that same period, which is notable since many of the schools had discipline issues, and attendance crept up by about 4 percent over the last four years.

De Blasio’s administration should have been quicker to replace principals in the schools that were taking the longest time to improve, according to the report, and should have avoided marking schools with a label that denoted poor performance, which scared off parents.

Renewal was created in part as a rebuke to former Mayor Michael Bloomberg’s plan to fix long-suffering schools, which relied on shuttering more than 100 schools and opening dozens of smaller schools with specific themes in their place, typically staffed by new principals and different teachers. Bloomberg put a heavy emphasis on using data to make decisions about schools, and de Blasio’s administration now seems to be heading in that direction.

Bloomberg’s small schools plan led to an increase in high school graduation rates and students going to college, all at a lower cost to the city than students in other types of schools, according to a series of studies. But while results were encouraging, the plan did not rescue all the schools it touched: More than a third of the schools that ended up on the original Renewal list were created during Bloomberg’s tenure.

Many advocacy groups and local politicians, including de Blasio, then the public advocate, railed against school closures. The city’s teachers’ union, the United Federation of Teachers, was the most powerful lobby against closures. The union’s president, Michael Mulgrew, who has a close ear to the ground in schools, has been critical of the Renewal program’s execution.

But because the program was based on the union-friendly theory that struggling schools need more resources, rather than more effective teachers, Mulgrew may face fresh questions about not only the execution but the design of the program.

De Blasio insisted Monday that developing a close relationship with the teachers’ union has led to a stronger school system. He said that ongoing conflict between Bloomberg’s administration and the UFT led to “paralysis and conflict,” and he pointed to the national turn away from education reform and toward teachers, as is evident in the recent wave of teacher strikes and growing political hostility toward charter schools.

“The era of closing schools has come to an end,” the mayor said.

This article originally appeared in The New York Times.