Debate flares again about school start times in Montgomery County, Md., where the school board voted in 2015 to push back the 7:25 a.m. opening bell of high school by 20 minutes. Pictured is a “sleep-in” held the night before the board vote. (Bill O'Leary/The Washington Post)

Nearly two years after Montgomery County leaders voted to start the school day later so teenagers could get more sleep, the debate that many thought was settled shows signs of making a comeback in Maryland’s largest school system.

Principals in elementary schools have been speaking out about the toll of schedule changes — drowsy children, longer bus rides, families strapped for child care. Employee unions are urging a return to the school hours of old, saying it would be best for students and staff. But parents who support later school days have weighed in, too, with many emailing letters backing the changes adopted in early 2015.

“Please do NOT revert to old bell times!!!” one high school parent wrote. “If anything I wish high school bell times were later.”

The issue came up briefly at a school board meeting in December and is on the agenda for the Jan. 10 meeting. “I think there have been some unintended consequences,” said Michael A. Durso, board president.

“We’ll look at it and discuss it and maybe there are some angles we haven’t thought of,” he added. “We’re just kind of exploratory at this point.”

The 2015 decision to reset the 7:25 a.m. opening bells of high school 20 minutes later — to 7:45 a.m. — followed more than two years of study and debate and was viewed by some elected leaders as a no-cost “first step” toward healthier school hours for teenagers.

Supporters have argued that later high school start times are in line with research showing that adolescents are biologically wired for later bedtimes and wake-ups, and that lack of rest is linked to increased risks of depression, car accidents and other problems.

But the changes at high schools affected schedules across the 204-school system. Elementary schools, which open in two waves, now start at 9 a.m. or 9:25 a.m., 10 minutes later than before, and their dismissals come 20 minutes later, so the length of the school day has been extended.

Principals at later-starting elementary schools have appeared at the board’s past three meetings to describe what they see as the fallout. Several talked about children being dropped off at school overly early by parents who need to get to work.

Dorothea Fuller, principal of Galway Elementary School, told the board that although the school’s PTA provides an after-school-activity bus so students can join in sports workshops, reading support, choir and other programs, fewer students do so because families worry about them arriving home by bus in the dark.

“With the change in bell times, student participation has dropped 30 percent,” she said.

Dina Brewer, principal at Sherwood Elementary School, said the change has led to a loss of instructional time, because children grow tired during a late-starting, longer day and buses frequently run late, delaying lessons.

Brewer recalled that on three occasions she received calls after 6 p.m. from teachers who happened to still be at the school when a bus returned with a child who had fallen asleep on the way home and was discovered at the end of the route. She said the school had never had students brought back so late.

“I’m sure you can appreciate the fear and panic that this caused,” she said.

Many teachers say they notice that elementary school students, especially the youngest, grow tired as the afternoons wear on, said Valerie Coll, a teacher at Flora M. Singer Elementary. “They tucker out,” she said. “Not all students are able to develop the academic stamina.”

The union that represents the county’s principals and administrators asked the board in November to return schools to the schedules they had in 2014-2015. The teachers union did the same, in a Nov. 2 letter to board members.

“We believe a return to the 2014-2015 bell schedule is in the best interest of our students and educators,” the letter said.

But a number of parents and advocates have weighed in strongly on the other side, saying the board should not scrap healthier start times for teenagers.

“That would be a horrible step backward,” said Mandi Mader, a Montgomery County parent who launched a 2012 petition that sparked the effort for change.

Mader said she and others welcome more discussion of the issue and would like to see a better solution for the most-affected elementary schools. They say they’ve always wanted healthy bell times for all, not just for teenagers. “Many districts have figured this out,” she said.

School board member Patricia O’Neill, 3rd District, said the school system’s choices have long been influenced by economics and transportation.

School buses travel multiple routes each day — so that a vehicle transporting high school students in the early morning is used later for middle-schoolers and later still for children in elementary grades. Not everyone can go to school at the same time, she said.

“This is a complex issue that has been studied and studied and studied over the years,” she said. “There is no perfect solution other than to buy more buses and hire more drivers,” which she sees as unlikely given budget constraints and needs such as addressing the achievement gap.

O’Neill said that complaints about some elementary schools starting too late preceded the change in bell times. Although she does not back a return to earlier hours at high schools, she would like to work toward a solution for the district’s later-scheduled elementary schools.

Elise Browne Hughes, a Bethesda parent and PTA leader, said the change at high schools has been better than expected.

Her teenage son has long struggled to wake up and was often a few minutes tardy for school, she said. She was skeptical about how much could be gained with a 20-minute shift. But she says his mornings are markedly better — and he gets to school on time. “It really did make a difference,” she said.

Experts say adolescents need 8½ to 9½ hours of sleep a night but find it hard to fall asleep before 11 because of their natural sleep cycles.

Eric Guerci, the student member of the school board, said that leaders of Montgomery’s countywide student government passed a resolution in December against returning to the bell times of the 2014-2015 school year. Although the current hours may not be perfect, he said, many see them as a benefit overall: “They’re seen as representing a positive change.”