RUTHFULLY YOURS

Here is what we know and don’t know about children and Covid-19, what measures schools in Europe are taking, and what we might find out.

What does science tell us about how children can become infected?

Anyone with children knows that the younger they are, the more likely they are to catch whatever pathogen they come into contact with. But the new coronavirus is different. Most doctors agree that children who catch Covid-19 rarely become seriously ill. How broadly they can spread the virus—and whether they are less susceptible to infections than adults—are still contested issues among scientists.

The World Health Organization has said early research suggests children don’t appear to be spreading the new coronavirus as often as adults, perhaps because younger patients rarely display severe symptoms and so tend to cough and sneeze less than older ones.

French Education Minister Jean-Michel Blanquer told The Wall Street Journal that the latest studies indicate that children below 10 are less contagious than those who are older.

However, Christian Drosten, head of the virology department at Berlin’s Charité clinic, last week warned about reopening schools after finding that a sample of infected children treated at his hospital carried the same viral load as adults.

A study published late April in the medical journal the Lancet that tracked transmission of the virus in Shenzhen, China, also concluded that while children are less likely to have severe symptoms, they are at a similar risk of infection to the general population and should be considered in measures to control the virus.

Another study of the outbreak—this time in the Chinese cities of Wuhan and Shanghai and the region of Hunan, published in the journal Science on April 29—found that children under 15 were about a third less susceptible to infection, with this difference becoming less marked for older children.

“We have to be prudent,” said Mr. Blanquer. “We’ve already seen there are evolutions in the scientific approach, which is inevitable for a new biological phenomenon.”

Doctors have also alerted their colleagues to an increase in the number of children suffering from Kawasaki disease, a rare inflammatory condition, in regions that experienced a large number of Covid-19 infections.

Should young children or older ones restart school first?

Virtually all European countries diverge on this issue.

Germany and Hungary have already reopened schools for older children who are taking exams at the end of the school year. Younger children in Germany will follow afterward, returning in waves up to the summer break.

France and Poland are doing the opposite, bringing back the youngest, including preschoolers, but only voluntarily. Norway opened elementary schools on April 27.

The Netherlands sits in the middle. It began opening schools on Monday on staggered timetables, with priority given to the first years of elementary school and the last years of high school.

Sweden never closed schools for children ages 16 and under.

In most cases, science is only one factor behind these decisions.

Young children are highly vulnerable to domestic violence or neglect, Mr. Blanquer said, which is one important reason to bring them back to school sooner. Likewise, he said, younger children have been a bigger burden on parents while older students need less supervision during home schooling. Reopening schools is a necessary step toward reopening the economy.

“We have to make choices,” Mr. Blanquer said. “More generally, we have to get society going again.”

One reason Germany is sending older children back first: to avoid the logistical headache—and emotional distress—of rescheduling end-of-year exams. Another is that older children can be better trusted to follow complicated hygiene and social-distancing rules or wear face masks properly.

Then there are private schools, some of which are eager to resume, out of concern that parents will stop paying fees. Public schools, by comparison, are under less pressure to restart

Given the risks of infection, should children return to school?

Initially, France and Poland will make sending children back to school voluntary. Some worry this could mean that those children most in need of returning would also be the least likely to do so. It is also demanding for teachers who now have to manage a classroom while holding online lessons for those who stay home.

Others, such as Germany, where school is compulsory and home schooling normally banned, have resisted changing the law—except for children with health issues that make them more susceptible to the disease.

In practice, many parents have resisted. In Denmark, the first European country to reopen schools, thousands kept their children home, fearing the schools would become hotbeds of infections. In Berlin, some parents have expressed concerns about children not having to wear masks in class.

Virginie Labourdette said she calculated that her 10-year-old daughter would have only five to six days of attendance at the school grounds before the summer vacation under the plan set up by her school to limit the amount of children present at the same time.

“I decided it wasn’t worth risking her health for just a few days,” said Ms. Labourdette, who lives in a suburb west of Paris. “We’ll re-evaluate in June.”

In the Swedish town of Monsteras, Cecilia Nilsson, a single mother, kept her 8-year-old son at home even though his school stayed open because she was afraid of getting sick herself and not having anyone to look after him.

Is there a consensus on hygiene rules for schools?

Yes. Students across Europe will be asked to refrain from physical interactions. This means fewer pupils per class, separate entrance and exit gates, staggered recess and lunch times, shifts throughout the day or on different days, and social distancing on school buses. There will also be a lot more handwashing and frequent disinfections of buildings.

Some countries go further than others. In France, government guidelines say parents should take their child’s temperature before sending them to school. The government also said there would be a limit of 10 children per class in preschools and 15 students per class in schools, while other countries are letting schools decide on the size of reduced classes.

In some countries, local politicians or schools have complained about difficulties in obtaining personal-protective equipment. In Belgium, where schools will reopen gradually from May 18, some regional officials warn that they will keep them closed if there aren’t enough masks and hand sanitizer.

Tiny but wealthy Luxembourg offered some 6,000 students of final classes and 2,500 teachers the option of getting tested before schools began opening gradually on May 4, beginning with those taking final high-school exams.

What about masks?

There is less consensus in Europe on masks.

In Germany, where states have enacted different rules, masks in schools are generally not obligatory. In Bavaria, in southern Germany, children and teachers are nevertheless advised to wear masks outside classrooms, such as in toilets, hallways and schoolyards or wherever else they can’t keep the necessary distance. There is concern that masks lose their effectiveness due to moisture after being worn for several hours.

In France, masks are being made available to teachers but are generally not being made mandatory. In elementary schools, students who start displaying symptoms of Covid-19 will be isolated and made to wear a mask until they can go home. Masks won’t be mandatory either in middle schools when they start to reopen on May 18 except in situations where distancing rules are difficult to respect.

Teachers in the Czech Republic and Poland have complained about the lack of public-health guidelines on whether students should wear masks in classrooms and, if so, for how long. There are no mask-wearing rules for schools in Sweden, where few people have worn masks in public.

In Belgium, children over the age of 12 and teachers will be required to wear face masks in schools when they reopen on May 18. In Hungary, police last week patrolled high-school campuses as teenagers took their literature exams, ordering students to keep their face masks on and maintain a distance.

Aren’t the rules too complex for children, especially small ones, to follow?

Complexity is a source of concern for many parents. In a small town southwest of Paris, Karen Mathers balked at the long list of rules her 4-year-old son would have to follow when returning to preschool next week: washing his hands every time he sneezes; not sharing toys or playing with classmates; sitting alone at a table to eat his packed lunch.

“I cried,” said Ms. Mathers. “I just can’t send him back like that.”

In Sweden, teacher Natalie Hogstrom recently helped out with a kindergarten class where 6-year-olds weren’t required to distance themselves from each other. “They put their hands in their mouth, and they all share pens and crayons,” she said.

Some countries, such as Germany, are exempting teachers whose health or age put them in high-risk groups from physical presence at school. With up to a quarter of all teachers in Germany belonging to this group, it is unclear how schools will be able to fulfill the goal of allowing all students to return to school in some form before the summer break, said Heinz-Peter Meidinger, president of the federal teachers association.

When will schools return to normal?

The same goes here as for any other part of life: Without a vaccine or an effective treatment for Covid-19, there will be no return to normalcy. Until then, authorities across Europe hope they can at least avoid a rebound in infections that would force schools to close again.

Some countries hardest hit by the virus, including Spain and Italy, have decided to leave schools closed for now.

In the U.K., which has suffered the highest number of deaths in Europe, British Prime Minister Boris Johnson said Sunday that a phased reopening of primary schools could begin June 1.

Karl Lauterbach, a member of Germany’s parliament and a professor of epidemiology, sparked outrage on social media last week when he predicted a return to normal in schools wouldn’t happen for at least another year because children, especially older ones, would prove to be carriers of the virus. “If this fall we allow big schools with maybe 2,000 students to operate normally then this virus will really kick off,” he said. “I know people don’t like to hear that.”

Ms. Hellmuth from the John-Lennon high-school in Berlin, said she and her teachers are preparing two plans for the new year: one normal timetable and one for a staggered system she expects will probably become the norm for some time even after the summer. And she is ready to face a new round of school closures if and when infections flare up again—an eventuality she sees as inevitable.

—Drew Hinshaw in Warsaw, Stu Woo in Linköping, Sweden, Valentina Pop in Brussels and Eric Sylvers in Milan contributed to this article.

Write to Ruth Bender at Ruth.Bender@wsj.com and Nick Kostov at Nick.Kostov@wsj.co