D.C. Schools Chancellor Kaya Henderson discusses the significantly higher standardized test scores achieved by DCPS students, and dismisses the notion that it’s because more middle class students are starting to attend public schools. (The Washington Post)

The District’s Tubman Elementary was a typical inner-city school. For years it struggled with uneven teaching, unruly behavior, and more than two-thirds of its students lacked proficiency in math and reading.

Then Tubman got a new principal, Harry Hughes, who in 2008 introduced a passel of changes just as then-Schools Chancellor Michelle Rhee was making national headlines for her efforts to overhaul D.C. public schools. He split older students into single-sex classes, expected teachers to arrive on time and work together and opened the building on weekends to volunteers who helped struggling readers.

The Columbia Heights school — where the vast majority of students qualify for free and reduced-price lunch — has made consistent gains in the past five years, more than doubling its scores on math and reading tests. Four out of five students are now proficient in math and 63 percent are proficient in reading, far exceeding the citywide average.

It’s a success story that exemplifies the school system’s strides, Schools Chancellor Kaya Henderson said last month, announcing a four-percentage-point gain on the city’s standardized tests — the largest average increase since 2008.

“The strategies that we have invested in are beginning to pay off,” Henderson said. “We have an in­cred­ibly high-caliber teacher and principal corps, and they are doing amazing work in our schools.”

Although the sweeping overhaul of city education has helped lift schools like Tubman, it also has left many city schools to struggle, especially in some of the District’s poorest neighborhoods. As city officials celebrate improvements and tout reforms, test data show that there are schools that have not benefited from the reforms and many that have fallen further behind.

Several elementary schools — Garrison in Northwest and Houston and Plummer east of the Anacostia River — saw double-digit declines in math and reading this year. Scores at Ballou and Dunbar high schools have slid in the past five years and fewer than one in five students at the schools are proficient in reading and math.

At Aiton Elementary in Northeast, scores have tumbled more than 40 percentage points in the past five years. Thirteen percent of students are proficient in math, according to 2013 test results, and 19 percent are proficient in reading. Half of students were “below basic,” the lowest possible category.

Standardized test results can be a blunt instrument for measuring schools affected by demographic changes and other factors that have little to do with teaching quality. But they are also a key indicator that officials use to judge schools, teachers and principals.

Citywide, about two dozen traditional schools — or one in five — recorded declines in math and reading this year. At more than half of those schools, scores were lower in 2013 than they were in 2008.

There’s no singular clear reason for the backslide, Henderson said, adding that officials will need to dig into the numbers more deeply to understand what they mean. But she suggested that — in a city where many of the toughest schools have seen frequent turnover of principals and teachers — strong and sustained leadership is among the most important missing ingredients. Like Hughes at Tubman.

“More than anything else, we need to get the right people leading our schools,” Henderson said. “Consistency of leadership is really important.”

Education experts universally agree that strong, sustained leadership is a key to improving struggling urban schools. And it is what many D.C. schools have lacked.

Rhee, who made headlines for firing principals who failed to meet her expectations, replaced leaders at 46 of about 120 schools in 2008, drawing criticism that her no-excuses approach was creating a new level of instability and churn. But while turnover certainly spiked that year, it has been high for decades, according to longtime D.C. education watchdog Mary Levy, who has kept track of principal changes since the 1990s. In general, about one-quarter of schools have opened each fall with new principals.

The turnover disproportionately affects schools with large numbers of poor students, which tend to employ the least-experienced principals and can be among the most challenging places to work. In 2010, one-third of the city’s high-poverty schools had first-year principals, according to a recent analysis by a consortium of education researchers from institutions including George Washington University. Only 5 percent of more affluent schools — those where fewer than 60 percent of students are poor — had principals with so little experience.

Aiton Elementary, for example — where 99 percent of students are poor — is one school that has struggled academically as it has churned through leaders.

The school made dramatic gains on 2008 tests, but the validity of those gains were called into question when the school was flagged for an unusually high number of wrong-to-right erasures on answer sheets. When test security tightened, Aiton’s fortunes declined sharply.

Last year, the school opened with a new principal who had transferred from a post in Montgomery County. He is not returning, and the school is slated to open its doors this month with its fourth principal in five years.

The leadership turnover has contributed to a broader instability that has directly affected students, according to a person familiar with Aiton who spoke on the condition of anonymity. One fifth-grade teacher, for example, quit a month into the school year last fall. Her students were sent to another teacher, who then quit around spring break. D.C. schools officials declined to speak specifically about Aiton.

Tubman, meanwhile, has had one leader for the past five years.

“I think part of the reason Tubman has made gains is because Mr. Hughes has been there continuing the work,” Henderson said.

Hughes, a onetime D.C. teacher, arrived at the school in 2006 as assistant principal and was named to the top job two years later. During his tenure, teachers and parents say, he has been an architect of school culture and instruction.

Hughes recalls that in his first speech to his staff, he told them that the school — with about one-third of students proficient in reading and math — was failing. “There was no way that you would want to send your child here,” he said. He promised that things would change.

First, he mandated that teachers come to work on time, which meant 45 minutes before the morning bell. Too many teachers were tardy, he said, and they needed that time to plan and solve problems.

Hughes redesigned the daily schedule to give teachers time to work together. He hired instructional coaches to help teachers parse test data to identify individual students’ strengths and weaknesses and then employ that data to design lessons that could meet those varying needs. And he made himself visible, popping into classrooms frequently and greeting children by name in the hallway.

Not all of the staff loved the changes, and some left. Hughes also took advantage of Rhee’s controversial 2009 layoffs to shed four staff members he deemed “poisonous,” including a custodian and a teacher whose students, Hughes said, spent an inordinate amount of time coloring and doing worksheets.

“The RIF [Reduction in Force] in 2009 is what I call addition by subtraction,” Hughes said. Since then, two Tubman teachers lost their jobs for poor ratings on annual job evaluations, he said.

Several teachers described the new Tubman as a friendly and respectful workplace where they have been encouraged to take on increasing leadership roles. The school is filled, they say, with teachers working hard, often for free, volunteering before and after school and on weekends to tutor kids in need of extra help.

“In the beginning it was kind of tough for some of the teachers, because it was a lot of time involved, but I think the teachers over the years became more invested,” said Monica Davis, who has been teaching at Tubman since 1994.

Hughes made structural changes meant to cut down on the misbehavior that made it hard for teachers to teach and hard for students to learn. One of his first decisions was to split third-, fourth- and fifth-graders into single-gender classrooms.

The change made an immediate impact on behavior, Hughes said, and Tubman has continued to separate older students into all-boy and all-girl classes whenever its rosters allow. Those students also rotate among teachers who specialize in math, reading or writing, and students take regular classes in art, music, physical education and science.

The school has largely maintained its student population — majority poor, about 55 percent Latino and 45 percent African American — even as the surrounding neighborhood has gentrified, bringing an influx of white and middle-class families.

Every Saturday for the past several years, dozens of struggling readers have flooded the school for the chance to read books with community volunteers, the kind of focused one-on-one reading that many children don’t regularly get at home.

The program, Reading All-Stars, is run by 826DC, the literacy organization founded by author Dave Eggers. It is nearly free, but it requires a principal who is willing to devote his Saturday mornings.

“On the morning when he and every other educator deserves to be resting, Harry Hughes shows up at school again,” Eggers wrote in a blog post after volunteering at the school.

Hughes received The Washington Post’s Distinguished Educational Leadership Award this year. In recommending him for the award, 11-year Tubman teacher Sheila Copeland described him as an “educational as well as moral compass” for his staff.

School system leaders say they recognize that they need more people who know how to lead urban schools. In January, officials launched an 18-month fellowship meant to train a dozen hand-picked employees to become principals.

Hughes, meanwhile, will not be returning to Tubman. For his success, he was rewarded with a promotion. He’s now an instructional superintendent overseeing 12 schools.

He said he has no doubt that Tubman, one of more than 20 schools slated to open with a new principal this fall, will continue to improve. “I’m confident that my strongest teachers can lead and run the school,” he said.

Valerie Jones, the parent of a rising fifth-grader, said Tubman has nurtured her son’s love of learning over the past five years. “It’s a real good school. I love it,” she said. “From the custodian to the teachers, the whole staff, everyone who works there, they work as a team to help the kids.”

She’s less thrilled about the principal’s departure. “That’s a good thing for him, but a bad thing for the students,” she said. “I’m sad to see him leave.”