Over the course of 20 days in March 2020, 55 million American children stopped going to school as Covid-19 swept the United States.

What was impossible to anticipate then was that millions of those students would not return to classrooms full-time until September 2021, a year and a half later.

Those children and teenagers, often in public schools in Democratic areas, remained online at home while private schools, child-care centers, public schools in conservative regions, office buildings, bars, restaurants, sports arenas and theaters sputtered back toward normalcy.

Five years on, the devastating impact of the pandemic on children and adolescents is widely acknowledged across the political spectrum. School closures were not the only reason the pandemic was hard on children, but research shows that the longer schools stayed closed, the farther behind students fell.

What would happen if another health crisis came along — a pressing concern, as cases of measles and bird flu emerge? In the face of a new unknown pathogen, how would school leaders and lawmakers make decisions?

“It’s so important for Democrats to do a retrospective on this episode,” said Representative Jake Auchincloss, Democrat of Massachusetts, who represents a district in the Boston suburbs where some schools were fully or partially closed for a year. He has argued that during the pandemic, his party “over-indexed” toward the views of teachers’ unions and epidemiologists, who often pushed for a slow, cautious approach to reopening schools.

The extended closures “crystallized how the party has been failing in governance,” Mr. Auchincloss said.

In some ways, moving to online learning would be easier next time, now that nearly all schools give students their own laptops or tablets. And in places where schools remained closed longer, some people in positions of power, including health officials and leaders of local teachers’ unions, say they stand by the decisions they made at the time.

Still, in interviews with more than a dozen leaders in health, education and politics, including some who were key figures at the time, others said they would take a different approach in the future, and try to do more to avoid extended shutdowns for entire school districts.

“Yes, I’ve learned a lot from this,” said Randi Weingarten, president of the American Federation of Teachers and a powerful force in Democratic politics who at times worked behind-the-scenes to negotiate reopenings. She also stood by locals in places like Philadelphia and Chicago, where union members fought for vaccines, tests, ventilation and other safety measures — even after classrooms in other parts of the country had reopened.

Ms. Weingarten defended her members’ right to work safely and emphasized the importance of ventilation, but said she would strive to be clearer in the future that “kids have to be the priority.” That includes in-person instruction, she said.

“I thought I was pretty loud,” she added. “I would be even louder.”

Conflicting advice in 2020

Few education or health leaders doubt that it was right for schools to close in March 2020, when much about Covid-19 was unknown.

But by early that summer, there was a spate of evidence that pointed toward a careful reopening. Classrooms had reopened abroad, with research showing that there was limited spread of the virus inside schools. It was becoming clear that children tended to be less severely affected by the virus than many adults were, and that young children were less likely to spread the disease.

The American Academy of Pediatrics issued a report in June 2020 recommending that schools reopen. Republican-run states like Texas and Florida forged ahead with plans to offer in-person instruction to families who wanted it.

Yet thousands of schools in Democratic-majority states like California, Oregon, Washington and Maryland stayed closed or partially closed for another full year.

Policymakers who had a role in those decisions argue that applying evidence from abroad was difficult because of several factors, including higher U.S. infection rates, less consensus around masking and limited availability of virus tests.

The politicization of the pandemic also played a role. President Trump repeatedly called on schools to reopen, while many Democratic officials and advocacy groups fought for stricter safety measures and more federal aid to schools.

In addition, conflicting advice from health experts caused confusion.

The Centers for Disease Control had, at times, recommended greater precautions than the pediatrics academy did, including maintaining six feet of distance between desks. In the summer of 2020, health agencies in states like California advised schools to remain closed in areas where case levels were high — which was almost everywhere.

The California Department of Public Health declined to respond to questions about their approach to school closures for this article.

Becky Pringle, president of the National Education Association, the country’s largest teachers’ union, said that following cautious public health guidance was the right approach, and is the one she would follow again.

“What we needed to do was to listen to infectious disease experts,” Ms. Pringle said.

She pointed out that rates of infection and death were higher in low-income communities of color, and that many parents preferred to keep their children at home.

“You try to make the best decisions with the information you have,” she added.

Sean O’Leary, a pediatric infectious disease specialist at the University of Colorado and the lead author of the academy’s report, recalled that some teachers pushed back against the report’s recommendations by pointing to crowded classrooms, dated H.V.A.C. systems and sealed-shut windows in their schools, many of them in low-income urban neighborhoods.

The teachers argued that reopening schools would be dangerous, and they organized marches — outdoors and masked — to demand that classrooms remain empty until virus transmission rates fell essentially to zero.

Dr. O’Leary said it was clear even at the time that those demands failed to consider what he called “the bigger picture.”

“What are the downstream consequences of closing schools?” he asked. “Is this the right decision as a society?”

Local officials who wanted to reopen schools sometimes found that their plans were superseded by governors and state health officials.

Heidi Sipe, the superintendent in Umatilla, Ore., a rural district that serves mostly Hispanic and low-income students, remembered releasing a video detailing a complex reopening plan for her district in the fall of 2020, only for the governor to announce shortly afterward that all Oregon schools would be remote that fall based on infection rates.

“It was devastating for us,” she said. “The challenge of that was the organizational trust that was lost — because so many of our families lost faith.”

Oregon’s school strategy mirrored a cautious approach to the virus more broadly in many Democratic states. “I’m proud overall of our response,” said Dr. Dean Sidelinger, Oregon’s state health officer, who noted that Oregon had one of the lower Covid mortality rates in the country.

But research now suggests that keeping schools closed was not a significant factor in slowing the virus, particularly after other parts of society were up and running. More people died in some Republican regions, Dr. O’Leary said, “not because the schools were open, but because they didn’t wear masks and didn’t get vaccinated.”

Would leaders make different decisions today?

Almost everyone in education acknowledges that extended school closures were damaging. Academic achievement plummeted and has not recovered. Student absence rates are double their prepandemic levels. And remote learning pushed children further into screens and away from learning and play in the physical world.

But even today there is not broad consensus about whether the lengthy closures were necessary.

Brent Jones, superintendent of Seattle Public Schools, said he was “not apologetic” about his system’s 18-month period of virtual and hybrid learning, one of the longest in the country.

“I saw it as a forced opportunity to step back,” he said. “We were called upon, frankly, to expand our mission to include many other things: nutritional, social, emotional, mental health. There was a cry for support. Schools stepped into that gap.”

Seattle also made investments in ventilation that he said could help keep classrooms open during another pandemic.

In some other cities, ventilation remains a sticking point, particularly in old school buildings.

“We would insist that the buildings be safe before they are occupied,” said Arthur G. Steinberg, president of the teachers’ union in Philadelphia, where dozens of school buildings do not have updated H.V.A.C. systems.

Still, he and others said that they would be more apt to consider school closures on a building-by-building basis, rather than pushing for systemwide shutdowns.

Stacy Davis Gates, president of the Chicago Teachers’ Union, acknowledged that some city schools “could have probably been fine” reopening sooner, but noted that they tended to be the ones in more affluent neighborhoods.

“How do you continue to create a policy that marginalizes people who have been marginalized for years?” she asked in an interview late last year.

Ms. Weingarten, the national union leader, said that in a future crisis, she would urge local unions to come up with their own safety plans, and to be creative in order to educate children in person — an approach many parents were desperate for during the Covid shutdowns.

If school buildings do not have proper ventilation, she said, “then you find other buildings within the city.”

Still, politics, not logistics, may be the biggest obstacle in a future health emergency.

Public trust in science and schools fractured during the pandemic and remains low, especially among Republicans. Governors and state leaders could once again split along partisan lines. If anything, over the last five years, Americans’ views about vaccines, public health and education have only become more divided and politicized.

Some of the mistrust seeded by the pandemic has spilled over into other arenas of education.

Debates about schools now often focus on how race, gender and American history are taught. Republicans are pushing new state laws to provide public money for families to send their children to private schools. The number of children nationwide who are using some form of private-school voucher has doubled since 2019, to more than 1 million.

Partisans on both the right and left say those trends might not have taken off without the widespread anger and frustration arising from how the education establishment handled Covid-19.

Public health experts caution that their guidance in a future health crisis would depend on the particular disease. A future pathogen could be far more dangerous for children or teachers than Covid-19 was.

“We don’t know what could be coming,” said Sean Bulson, the superintendent of schools in Harford County, Md., outside Baltimore. But based on what was learned over the past five years, he said, “our threshold for closing probably got higher.”