Anacostia High School graduates celebrate during their commencement ceremony in June 2010. (Jahi Chikwendiu/WASHINGTON POST)

Less than 60 percent of D.C. high school students graduated on time in 2011, according to a new and more rigorous calculation of completion rates announced Thursday.

Figures released by the Office of the State Superintendent of Education show that 58.6 percent of students in the Class of 2011 obtained high school diplomas within four years. That’s a nearly 20 percent decline over the 73 percent rate reported for 2010.

The new numbers also revealed a widening gap between the city’s public charter schools and traditional public high schools in the ability to graduate students on time. Eight in 10 charter seniors received diplomas last year, compared with slightly more than half of those in traditional schools.

D.C. officials said they were prepared for the overall drop-off. States and school systems across the country have seen similar slides since the federal government required in 2008 that they tighten up their counting methods, which often involved simply dividing the number of students receiving diplomas by the number of those who started ninth grade four years earlier. The new rules call for schools to track individual ninth-graders and follow them if they move.

In Montgomery County, for example, the new “cohort” method pushed graduation rates from 90 percent to 86.7 in 2011. Fairfax County, which has been using the new calculus for several years, reported a 91.4 percent completion rate in 2011.

District educators said the new numbers, while not revelatory, were another stark reminder that they have a long way to go in preparing a huge number of high school students for college and the workplace. The D.C. school system has employed a number of different programs to identify and support students at risk of dropping out. Last month, the D.C. Council passed legislation sponsored by Chairman Kwame R. Brown (D) that would establish “an early warning” system to spot at-risk middle and high school students.

“For years, we’ve known that our graduation rates did not accurately reflect our successes,” Schools Chancellor Kaya Henderson said in a statement. “With the new calculations, we have a clearer understanding of the work we still need to do, and the public has a more reliable way to hold us accountable.”

Of the 29 D.C. schools that serve high school students, the city’s four selective, application-only schools — Banneker, McKinley, School Without Walls and Duke Ellington — are at the top of the revised list. One hundred percent of Banneker’s students graduated on time. The other three had rates between 91 and 93 percent.

Of the city’s open-enrollment high schools, the top seven were publicly funded, independently operated charter schools: Washington Math, Science & Technology; SEED; Booker T. Washington; Friendship (Woodson campus); Cesar Chavez (Capitol Hill and Parkside campuses); Perry Street; and Thurgood Marshall Academy. The rates ranged from 91.3 percent (Washington Math, Science) to 75.4 percent (Thurgood Marshall). The traditional open-enrollment public high school with the highest graduation rate is Woodrow Wilson in Northwest, where 73.7 percent of the Class of 2011 finished on time.

Seven of the 10 high schools with the city’s lowest rates — 60 percent or less — are traditional public schools: Dunbar, Coolidge, Woodson, Ballou, Spingarn, Roosevelt and Cardozo. The eighth of the 10, Anacostia, is a D.C. public school is operated by Friendship Public Charter Schools under a contract with the city.

Overall, charter schools outperformed traditional public high schools in on-time graduation, 79.7 percent to 52.9 percent. That differential is larger than the 2010 totals calculated under the old formula, when charters showed an 86.6 percent graduation rate compared with 75.5 for traditional public high schools, according to the state superintendents office.

Scott Pearson, executive director of the D.C. Public Charter School Board, said the performance of the charter high schools was “a testament to the hard work of all D.C.’s charter school leaders.”

“D.C.’s public charter schools serve a high percentage of at-risk students, many of whom will be the first in their families to attend college,” Pearson said.

But others said that the two school sectors operate differently and that traditional public schools have less control over determining their student bodies. Charter schools, untethered from collective-bargaining agreements and the municipal bureaucracy, are also free to extend their school days and pursue other innovations.

“It’s not a totally level playing field,” said Mary Lord, a member of the D.C. State Board of Education. “Charter schools use lotteries, and DCPS schools are by definition the default schools so they have to take all comers.”

Lord said the divide in graduation rates reflects “the challenge of educating kids in high poverty neighborhoods writ large.”

Robert Balfanz, director of the Everyone Graduates Center at Johns Hopkins University, which has tracked and reported graduation rates annually since 2002, said the city has a steep hill to climb.

“They have a huge problem,” he said. “There’s no work in the 21st century if you don’t have a high school diploma and no way to support a family.”