The first sign that something was wrong at the lone school in the tiny Belgian hamlet of Sibret was when a teacher began to feel sick not long after classes resumed this month. She tested positive for the coronavirus. Within days, 27 students and five other teachers also tested positive.

Now the village of 800 in Belgium’s rural southeast corner has become one of the latest data points in a complicated, angst-ridden experiment for communities around the world: How much does in-person schooling contribute to the spread of the virus?

The answer, experts are saying in Europe after several weeks back in classrooms, is that it’s rare for children to spread the virus within the walls of a school, but not unheard of. Not every country can point to a school where the coronavirus seems to have spread. And even where there are such schools, including in Belgium, Norway and Germany, such outbreaks typically remain countable on a single hand — affecting a fraction of a percentage point of the millions of students and teachers in session across the continent.

So despite rising coronavirus cases, and although universities have emerged as sites of concern, European countries remain wholeheartedly committed to in-person learning for primary and secondary schools.

“It is clear that children can pass on the virus to each other. It’s not that this doesn’t exist,” said Steven Van Gucht, the head of viral diseases at Sciensano, Belgium’s national public health institute. But in the weeks since Belgian schools resumed on Sept. 1, he said, few had triggered any cause for concern. Of Belgium’s 8,400 schools, 16 have closed fully or partially because of the coronavirus. That’s less than 0.2 percent of the country’s schools, and most closed due to staffing shortages after teachers contracted the disease in the community, Van Gucht said — not because the coronavirus spread beyond the initial person who got sick.

Viral spread in school appears rare enough, he said, that Belgian policymakers think having in-person classes might actually be safer than virtual schooling, assuming students tend to be less rigorous about social distancing when they’re not being supervised in classrooms.

“The school environment, in our perception, is still quite a controlled environment,” he said. “We think it’s better to have schools open than to send kids home, have them meet on the street and give them more opportunities to spread the virus.”

Many countries in Europe have dropped rules about wearing masks in schools, reasoning that it’s difficult for students to concentrate when they have them on all day. Public health authorities have spent more energy devising ways for children to study within relatively small cohorts, so that if quarantines are required, fewer people will be affected.

Still, contact tracing can be difficult. When multiple cases have popped up within a class, health authorities have sometimes found it challenging to determine whether transmission happened in the context of school or elsewhere, such as during playdates or community gatherings.

Media coverage has sometimes clouded understanding, too, with a focus on the number of students asked to quarantine rather than on the number of infections or their origins.

In Finland, more than 2,700 students and teachers were told to quarantine after being exposed to coronavirus cases since schools resumed in mid-August. But fewer than 10 people are believed to have contracted the virus after the initial exposure, said Otto Helve, a specialist in pediatric infectious diseases at the Finnish Institute for Health and Welfare.

“It’s important to have transmission control measures in place in schools,” he said. With such measures, he said, outbreaks can be limited.

One outbreak that has attracted significant scientific scrutiny in Europe is a June episode in Norway, where a total of 40 people, including 16 students, at the Sagdalen primary school in Lillestrom were infected. Through genetic analyses of the cases, investigators determined that the coronavirus likely was introduced to the school outside Oslo by two different people at roughly the same time.

One of the sources was probably an adult at the school, who infected other adults and children. The other source was likely a child, who may have infected other children at the school, but investigators said the children also had close contact outside school.

“It’s not easy to say that the children spread it,” said Margrethe Greve-Isdahl, a senior physician at the Norwegian Institute of Public Health, who is responsible for Norway’s guidelines to prevent infections in schools. “If asymptomatic transmission was common, we should be seeing a lot more cases than what we’re seeing right now.”

She noted that Norway had decided as a society to prioritize in-person education.

“The view in Norway is that children and youth should have high priority to have as normal a life as possible, because this disease is going to last,” she said. “They have the lowest burden of the disease, so they shouldn’t have the highest burden of measures.”

The thinking is similar in Switzerland. There, classrooms are quarantined only after a first coronavirus case is followed by a second one, suggesting the disease might have spread. Only a few classrooms in the country have needed to close under this rule, according to Jürg Utzinger, the director of the Swiss Tropical and Public Health Institute.

“Children are not identified as the main driver of the pandemic,” he said.

In Belgium, at Sibret’s community school, “it was a surprise for us to see so many cases,” said Yves Besseling, the mayor of Vaux-sur-Sûre, which encompasses Sibret and some of its neighboring communities.

The school isn’t certain how the virus spread. One teacher learned on Sept. 8 that she was infected, Besseling said. Another teacher tested positive three days later, as did a student in the preschool grades. After a third teacher fell ill, authorities decided to test all 120 students.

“When we decided to test the entire school, everyone was acting reasonably,” Besseling said. “But because a lot of the tests turned positive, we had no choice than to close the school.”

In all, 27 students and six teachers tested positive, the mayor said. Almost all the children were asymptomatic. The teachers had mild symptoms.

The mother of one of those children said her family had accepted that in-person schooling would be a risk — and wanted the children in school anyway.

“We are not panicking. There is no need,” said Aline, who asked that her family name not be used to preserve the privacy of her children. Only one of her two children at the school tested positive, she said, and has a mild fever. The rest of the household was negative.

“We hope actually to be all positive, to get through this, and so we can relax and move on,” she said. “My kids think they are on vacation. So they are fine. They’re playing in the garden.”

In Germany, one of the first countries to send students back to class this term, the daily number of new infections reached 2,500 over the weekend — the most since April. Still, public health experts say school openings have been a success so far.

The newspaper Die Welt surveyed the nation’s federal states and found that 49 out of 33,000 schools — about 0.15 percent — had gone into full quarantine since the beginning of the school year. In almost all cases, authorities do not believe that the disease spread at school.

“The schools are not driving this,” said Tobias Kluth, the director of Charité’s Institute for Public Health in Berlin. “The schools are a mirror of what’s going on in society.”

At one Hamburg high school, 36 students and three teachers tested positive this month. The state health authority said there is “much to suggest” that the virus spread among the pupils at school “but there is still no scientific evidence,” because investigations are still ongoing. The infected pupils ranged in age from 12 to 14. At another school in the town of Giessen, near Frankfurt, 13 students in ninth grade and another in the 11th grade tested positive.

The approaching winter remains a concern, and some are critical of Germany’s efforts so far.

“The schools are the weakest point in our preparation for a second wave,” said Karl Lauterbach, a member of parliament with the center-left Social Democrats and an epidemiologist. Germany has not done enough to prepare resources for digital instruction or try to look for space for additional classrooms, he said, and air filter machines were deemed too costly by school districts. Teachers are encouraged to air rooms frequently by opening windows.

“Fresh air is available now,” he said, but as temperatures drop across Europe, “classes with open windows will not be sustainable in a few weeks.”

Birnbaum reported from Riga, Latvia. Morris reported from Berlin. Ariès reported from Brussels. Luisa Beck in Berlin contributed to this report.