The school day is about to get longer for most middle school students in Prince George’s County.

“What?!” one shocked student at Oxon Hill Middle School said when she and some friends learned of the plans.

Starting in August, many middle school students will receive up to 40 additional minutes of help in science, math or reading. Others, who don’t need remedial instruction, will get an equal amount of enrichment time in subjects such as music or foreign language.

Longer days, which in this case will require no additional funding, are part of a national movement to raise academic achievement and a key element in D.C. Schools Chancellor Kaya Henderson’s plan to improve the city’s schools.

Experts say that a longer school day is no guarantee of better academic results and that educators must think hard about how to use the additional minutes.

Prince George’s officials promise the extra time will not be just more of the same.

“If you extend the time to do what you already do, you are not going to be productive,” said Superintendent William R. Hite Jr. “But if you extend it and engage students, offer problem-solving opportunities, reasoning and critical thinking, that produces student outcomes.”

Middle school students in the Washington area’s third-largest school system spend six hours and 40 minutes in school, ranking near the bottom statewide in the number of hours spent in school, according to Maryland data. They attend school 180 days a year, the state minimum. Montgomery County middle schoolers go to class 184 days a year and stay in school for six hours and 45 minutes a day. Fairfax County middle school students are in school for six hours and 50 minutes a day and their D.C. counterparts for 61 / 2 hours.

Under the new plan, Prince George’s middle school students will be in school as much as seven hours and 20 minutes a day, longer than most of their peers in the region.

Officials said the new schedule will not only improve student achievement but also reduce transportation costs. The extra class time is actually a byproduct of a plan to merge bus routes in a county with more than 123,000 students.

Under a route-sharing plan, which officials say will save the system $5 million a year when it is fully in place, middle school students will ride buses with high school students, which means the middle schoolers will either start their day 40 minutes earlier or return home 40 minutes later. The number of bus routes for middle and high schools will drop from 2,135 to 1,710, according to school officials. The plan also requires some elementary school students to be in school an additional 10 minutes.

Some middle schools that are adding sixth-graders because of boundary changes will not be affected immediately, Hite said.

The superintendent said he wants to expand learning time in elementary and high schools. To do that would require more money, and Hite said the system can’t afford it.

Kenneth Haines, president of the Prince George’s County Educators’ Association, said the teachers union contractually has no say in Hite’s decision to change the school day. The school system does not have to pay the teachers more because part of the extra time will come from their planning period.

Haines said teachers’ opinions about the change vary. “There will be some who are unhappy and some who are happy,” he said.

At Oxon Hill Middle, the school day now runs from 9 a.m. to 3:40 p.m. But starting in August, the day will end at 4:20 p.m.

Principal Wendell Coleman said that exactly how the minutes will be used remains to be decided. One option is to add an extra period. Another is to lengthen each class.

Kevin Thompson, an eighth-grade reading teacher, said additional minutes would enable him to analyze data on student progress in more depth.

Destiny Richardson, 13, a seventh-grader, said the new schedule could mean she will get home close to 7 p.m. on days when she has after-school choir practice and tutoring for reading. Richardson sometimes gets home at 6 p.m. under the current timetable.

“It’s an okay idea because I know a lot of people need the help,” she said. “The people who don’t like school will probably have an attitude.”

Another student said the additional time could create “more conflict” in the school. “Kids at our age have short tempers,” he said.

Still another said, “Some might even slack off.”

Shelita Campbell-Bennings, an eighth-grade science teacher, said she anticipates the additional time will give her a chance to see whether students are grasping what she is teaching.

She said class sometimes ends before she can sufficiently review the material. Having more minutes in the day, she said, “would give me more time to see if kids understand it.”