Earlier this year, the spread of the novel coronavirus forced more than 1.5 billion students into hastily implemented experiments in learning from home as schools around the world shut their doors.

More than half a year into the pandemic, a global experiment is underway that could prove even messier: the return to the classroom.

President Trump this week applied mounting pressure on state and local authorities to have schools resume in-person classes in the fall, although many experts and officials say it would not yet be safe, as case numbers remain high in much of the country.

In a tweet Wednesday, Trump suggested that other countries, including Germany, Denmark, Norway and Sweden, had opened schools safely.

Though some students around the world did begin to return to the classroom in May, more than 1 billion remain affected by school closures, according to data compiled by the United Nations Educational, Scientific and Cultural Organization.

Though he has been vocal, Trump has limited say in when schools reopen, and few countries that have sent students back to school in person have outbreaks that match the scope of the one in the United States.

But many countries are moving forward with reopening, as debates unfold about how to weigh disruptions to a child’s education and the costs of child care for those returning to work against health risks.

Many countries hope to resume in-person classes in the fall

Some nations have reopened their schools already, citing evidence that their outbreaks are under control.

Many have issued new safety requirements. At the epicenter of the initial outbreak in Wuhan, China, schools reopened in early May, but children had to pass through temperature checks, wear masks, and enter and leave at specific times to avoid crowding.

Denmark, one of the countries cited by Trump, closed its schools on March 11. Just a month later, it became the first country in Europe to reopen them, with almost all primary schools operating by April 20. In Sweden, classes continued uninterrupted for children under the age of 16.

In many nations, the vast majority of students are expected to be back for in-person classes when the new school year begins in late summer. Belgium, Britain, France and Italy are all pushing for plans to reopen schools widely in September.

Germany’s federal education ministries reached an agreement in June that should see students return for the new school year, with a required five-foot minimum distance between students in classrooms, the rule that made it impossible for classes to resume at full capacity before summer vacations.

Most of these nations are not seeing a surge in cases like the United States is. But Russia, which has the fourth largest number of cases in the world, announced on June 29 that schools in the country would open their doors on Sept. 1 as usual.

Some countries remain undecided

Many other nations are unsure whether they will reopen schools anytime soon. In India, home to the third largest number of coronavirus cases in the world, students are already back from their summer break, but schools remain closed. Students will likely return to the classroom at different times, state by state.

Brazil, which has the second largest number of coronavirus cases in the world, is taking a similar patchwork approach. Some private schools reopened this month, and the country, which has no education minister at present, appears to be allowing local governments to call the shots.

In Israel, as new cases surge again, the central government appears to be delaying any decision on reopening in September. And in Kenya, schools are not expected to reopen until early next year — the regularly scheduled start of the academic year in the country.

No primary and secondary school examinations will be held this year and “the 2020 school calendar year will be considered lost due to covid-19 restrictions,” Education Minister George Magoha told the BBC on Tuesday.

Return to school does not mean return to normalcy

Though the resumption of in-person schooling can help children’s education remain on track and allow parents to work, it also presents many problems for how to limit the spread of the coronavirus.

Some schools have pushed forward by mandating safety equipment like face masks, sanitizer and desk shields, but not all countries have been able to implement these measures. Others have taken to limiting class sizes or staggering arrival times. Many have allowed only graduating classes to return to school at first.

Risks remain, and some parents have refused to send their children back to school. In some countries, including South Africa, the fiercest opposition to school reopening comes from teachers, who argue that a crowded classroom would put them and their students at risk.

Some countries where school reopened early have seen a new wave of local closures as cases resurge. After recording no new cases from April 30 to May 22, the Japanese city of Kitakyushu in Fukuoka prefecture recorded 119 confirmed cases in just 11 days — including 11 pupils from four elementary and junior high schools — prompting an order to re-close schools.

In South Korea, an initial late-May reopening was delayed by a week after an outbreak in Seoul’s nightlife district of Itaewon. And just days after the first students set foot back in the classroom, hundreds of schools were closed after a sudden spike in new cases.

In China, Beijing authorities once again shuttered schools in mid-June during a new outbreak in the capital city. Chen Bei, Beijing’s deputy party chief, said the city needed to take “decisive action.”