Tanya Roane, principal at Francis L. Cardozo Education Campus, where graduation rates increased last year. (Nikki Kahn/The Washington Post)

The percentage of high school students who graduated from D.C. Public Schools in four years increased by six points for the Class of 2015, reaching 64 percent, a significant boost after several years of incremental growth.

A closely watched statistic in the District — and one that city leaders have vowed to improve — the graduation rate still rests well below the national average of 81 percent. But city officials said the graduation surge shows that their investments in the city’s public school system are paying off.

The city funneled millions of dollars of additional resources for high school electives and college-level courses this year, and the school system has for years been working on organizational improvements for tracking students in the highly mobile school system and monitoring their progress toward graduation.

“We are focused on preparing our students for future success in college and in their careers,” D.C. Schools Chancellor Kaya Henderson said in a statement. “Our graduation rates show that we are making real progress with students across the District.”

Seven of the city’s 13 comprehensive, adult or alternative high schools increased their four-year graduation rates from the previous year. Three schools — Francis L. Cardozo Education Campus, Paul Laurence Dunbar High School and H.D. Woodson High School — had double-digit gains. The city’s six high schools requiring applications recorded smaller changes.

Woodson increased its graduation rate by 10 percentage points, to 70 percent. That followed a 16-point increase from the year before.

The improved rate at Woodson in a low-income neighborhood in Northeast Washington puts the school above the citywide average and just five points shy of the graduation rate for Woodrow Wilson High School, the District’s largest comprehensive school, located in an affluent part of town.

Woodson Principal Darrin Slade credited a sea change at the school, including a reduction in suspensions, improved attendance and an individual focus on every student’s progress toward graduation. “Our focus is on graduation and promotion,” he said. “When you walk into school, that is all we push.”

Slade said the numbers also reflect improved recordkeeping, with fewer students falling through the cracks when they stop coming to school. For example, he said he went to the central office for Prince George’s County Public Schools to check whether students had reenrolled in schools there; if they had reenrolled, they would not count as non-graduates at Woodson.

Systemwide, African American male students had a seven-point gain, while Hispanic male students had a two-point increase. Graduation rates increased by more than five points among special education students and ­English-language learners.

The numbers are preliminary, which means that schools can appeal their graduation rates with the D.C. Office of the State Superintendent of Education. Final numbers for all schools are expected to be released this fall.

The D.C. Public Charter School Board did not report graduation rates for individual schools, but it said that early numbers indicate an overall two-point increase from last year, to 71 percent for all charter high schools.

In Virginia, more than 90 percent of students who entered high school four years ago graduated on time in 2015, according to state data released Tuesday. That represents an increase in the on-time graduation rate for the fifth year in a row, to 90.5 percent.

Steven R. Staples, the Virginia state superintendent of public instruction, noted that graduation rates rose even as state standardized tests, which students have to pass to graduate, have gotten more challenging.

“That we’ve seen another rise in the graduation rate — despite a significant increase in the expectations for high school students — indicates the hard work and professional expertise of the teachers, principals and other educators in the commonwealth’s high schools are making a real difference,” Staples said in a statement.

Most Northern Virginia school districts continue to outpace Virginia’s statewide graduation rates. Among large county districts, Loudoun had the region’s highest rate, at 95.6 percent. Arlington (92.8 percent), Fairfax (92.7 percent) and Prince William (91.4 percent) counties all beat state averages. Among cities, Alexandria, which has only one high school and a high concentration of ­English-language learners, had a graduation rate of 79.6 percent; Falls Church, a district of about 2,500 students that had 168 graduates last year, had a graduation rate of 98.8 percent.

In Virginia and in the District, four-year graduation rates are calculated by following a cohort of students who started as freshmen and graduated with a diploma four years later. It accounts for students who transfer in or out of a given school.

The rate reflects the number of students earning degrees but also, importantly, how well schools track down students who leave, because a school must show that a student has reenrolled in another school, whether across town or in another country. Any students that schools cannot track down are counted as non-graduates.

Michelle Lerner, a spokeswoman for D.C. Public Schools, said that administrators are improving their processes for obtaining the proper paperwork for students who transfer in and are making sure students are enrolled in courses that lead to graduation. The school system also is working to provide credit recovery to students who fall behind.

Dunbar High School charted a 10-point increase in the 2014-2015 school year, to 65 percent. Former principal Stephen Jackson credited the school’s ninth-grade academies — started four years ago to provide intensive support for a cohort of incoming freshmen — a model that has since spread to other high schools. He also cited “twilight” academies, in which students who are overage or behind in credits can make up past courses online.

He said the school also stepped up its efforts to track down missing students: “We would go to their homes, call all the known numbers, write to them.” A team of counselors and administrators rented vans and picked up former students and helped them enroll in other programs, he said.

Tanya Roane, principal at Cardozo Education Campus, where the graduation rate increased by 11 points, to 52 percent, said school administrators also are working much harder to track down students who left. The Columbia Heights campus serves a large immigrant population, and many students who leave go to another country, complicating efforts to find them.

“It’s very time-consuming work, but it makes a huge difference in the graduation rate,” she said.

The school also assigns adult mentors at the beginning of the year to work with a small number of seniors, making sure they stay on track and providing help when they veer off course, including tutoring, home visits with parents and credit recovery.

“Improving graduation is a heavy emphasis district-wide,” Roane said. “All of us principals are sharing ideas for what works.”

Clarification: An earlier version of this story referred to Loudoun County as having the highest graduation rate in Northern Virginia. While Loudoun does have the highest rate among the region’s large county districts, Falls Church City had a higher graduation rate. The story has been updated.