EIGHT SCHOOLS in the D.C. public school system last year started experimental programs built around a longer school day. Students at seven of the schools posted gains in both reading and math as shown on recently released test scores; four schools showed double-digit growth. The results are yet more evidence of the effectiveness of a longer school day in boosting student achievement; hopefully, that will spur expansion of extended hours to more schools.

Data from the 2013 Comprehensive Assessment System showed significant gains in test scores at schools where instruction isn’t cut off at 3:15 p.m. but continues until 4:15 or 5 in the afternoon. Gains at these schools exceeded those at schools with the traditional schedule by an impressive margin: 10.6 percentage points to 3.3. percentage points in math and 7.2 percentage points to 3.7 percentage points in reading. “We simply felt we had to have more time with our children . . . if we were to catch them up, we had to have them longer,” said Kennard Branch, principal at Ward 8’s Garfield Elementary School, where math scores increased 13.2 percentage points over last year and reading by 6.2 percentage points.

Garfield was one of eight schools that used a “Proving What’s Possible” grant to build 90 more minutes of instruction into the school day. Key to the success at Garfield and the other schools was not simply doing more of the same but coming up with new strategies and interventions tailored to the specific needs of students.

School officials told us they knew even before the test scores were posted that the longer school day — cited by effective public charter schools as a factor in their success — was having beneficial effects, so plans were made to extend the program to two more schools this fall. Schools Chancellor Kaya Henderson would like to extend it even further, but she must contend with a contract with the teacher’s union that limits the workday to 7.5 hours. Teachers at the schools piloting the longer days had to vote to work beyond the contract hours and are paid for the extra time.

A new contract is being negotiated, and school officials were said to have been close to an agreement on a longer school day. But the president who negotiated that tentative agreement was voted out of office. It’s unclear what Elizabeth Davis, the newly installed president, will do. She has expressed skepticism about longer school days.

Money probably will also be an issue, but the money the school spends on after-school programs might better be leveraged to lengthen the school day and allow for enhanced instruction.

There can be “no doubt,” to use the words of Garfield’s principal, that everyone — teachers, parents, children — benefits when children have more of an opportunity to learn.