On the West Coast of the United States, the region with the most American infections so far, Los Angeles declared a state of emergency on Wednesday, advising parents to steel themselves for school closures in the nation’s second-largest public school district. Washington State, which has reported at least 10 deaths from the outbreak, has closed some schools, while on the other side of the country in New York, newly diagnosed cases have led to the closure of several schools as well.

The speed and scale of the educational tumult — which now affects 290.5 million students worldwide, the United Nations says — has little parallel in modern history, educators and economists contend. Schools provide structure and support for families, communities and entire economies. The effect of closing them for days, weeks and sometimes even months could have untold repercussions for children and societies at large.

“They’re always saying, ‘When can we go out to play? When can we go to school?’” said Gao Mengxian, a security guard in Hong Kong whose two daughters have been stuck at home because school has been suspended since January.

In some countries, older students have missed crucial study sessions for college admissions exams, while younger ones have risked falling behind in reading and math. Parents have lost wages, tried to work at home or scrambled to find child care. Some have moved children to new schools in areas unaffected by the coronavirus, and lost milestones like graduation ceremonies or last days of school.

“I don’t have data to offer, but can’t think of any instances in modern times where advanced economies shut down schools nationally for prolonged periods of time,” said Jacob Kirkegaard, a senior fellow at the Peterson Institute for International Economics in Washington.