The number of students being educated in American schools dropped by nearly 3 million in the last year to the lowest share in more than two decades as the coronavirus pandemic sent families looking for new arrangements.

The decline hit hardest among both the youngest children in schools and the oldest: Just 40 percent of 3- and 4-year-olds were enrolled in school in 2020, the first time since 1996 that fewer than half of those children went to school. At the same time, college enrollment levels fell to their lowest nadirs since 2007.

The new data, released by the U.S. Census Bureau this week, is a worrying sign to some experts that the pandemic’s impact is going to be felt long after the virus itself recedes as a threat. Students who stay out of school miss valuable neurological and social development opportunities, some of which cannot be made up once they are lost.

“The enrollment data and the declines in particular are giving us multiple signals about the magnitude and character of the disruption in kids’ educational trajectories,” said Thomas Dee, a professor at Stanford’s Graduate School of Education who has studied the impacts of the pandemic. “Many kids are missing what are thought to be many key developmental experiences.”

The data does not show where the unenrolled children went, though there are other signs that more parents than ever are homeschooling their kids. 

But it does show that millions of children were no longer enrolled in preschool, elementary or secondary schools, either public or private, in October 2020. An estimated 4.7 million 3- and 4-year-olds are not enrolled in school, and 2.5 million children between the ages of 5 and 17 are not in school.

In the previous survey, conducted in 2019 before the pandemic broke out, 3.7 million 3- and 4-year-olds were not enrolled in school, and just shy of 1.8 million children between the ages of 5 and 17 were not enrolled.

A spokesman for the Department of Education did not respond to a request for comment by press time.

Data from the National Center for Education Statistics, released in July, shows enrollment in public schools during the 2020-2021 school year fell by 3 percent from the previous year. The drops were steepest in New England states and states like Washington and Michigan, which implemented some of the strictest lockdowns during the pandemic. New Mexico, Kentucky and Mississippi all experienced among the steepest declines as well.

Some education experts said they were less concerned about a steep and prolonged decline in learning by the youngest generation, in part because American education systems build in redundancies and repetition in curriculum. 

The decline in college enrollment in particular offers a reason for optimism, said Jennifer Steele, a professor at American University’s School of Education. The data shows a drop-off reflected in October 2020, a year ago, before vaccines were widely available and when many campuses were shuttered. A year later, the situation is much different.

“Campuses were largely closed, so some students did choose to wait a year to matriculate,” Steele said. “What we’ve seen at American University is those students have come back this year.”

Steele also said the pandemic that forced millions of American students and parents to rely on virtual learning would bring positive developments in the future, by showing educators and administrators that virtual options can work in some instances. Parent-teacher conferences or school functions are now accessible on smartphones, tablets and computers, giving parents who might have to work or cannot otherwise get to school the chance to participate.

“This move toward using virtual when it makes sense is a win for educational equity,” Steele said.

Remote learning presented the largest challenge to the youngest cohorts, contributing in part to their declining enrollments, Stanford’s Dee said. 

On Capitol Hill, lawmakers said the declines were concerning enough that they warrant amendments to public health procedures in future crises that are certain to come.

“It is clear that Covid disrupted many aspects of society, including education,” said Rep. Virginia Foxx (N.C.), the ranking Republican on the House Education and Labor Committee. “We will have to see if this is a short-term issue or a longer-term trend. In either case, we must ensure policies allow families to send their children to the education environment that best fits their needs.”

A spokeswoman for House Education and Labor Committee chairman Bobby Scott (D-Va.) pointed to a provision in the American Rescue Plan that sets aside 20 percent of funding allocated to education systems for addressing learning loss. States were required to show the Department of Education plans on reengaging students who fell by the wayside in the pandemic year.

It is too early to know just how disruptive the school closures wrought by the pandemic have been, or will be, to the development of those who would otherwise have been enrolled. Education analysts will be monitoring test scores and other metrics for years to come.

But by one benchmark, the early returns are troubling: The National Assessment of Educational Progress (NAEP) showed that for 13-year-olds, math and reading test scores dropped sharply compared to 2012.

Those test scores may rebound over time — the NAEP has showed steady progress over decades, as American schoolchildren of today score higher than their parents did at a similar age. And next year’s count will show how many of those youngest students stayed out of school even as the threat of the pandemic recedes and in-person learning returns.

Until those numbers rebound, however, some will remain worried that the true toll of a year and a half of learning disruption would pose a long-term toll.

“It’s a leading indicator of the educational disruptions due to the pandemic,” Dee said. “There’s enough here to be concerned.”