There was once a time when America’s lowest-performing students were improving just as much as the country’s top students.

Despite their low scores, these students at the bottom made slow but steady gains on national tests for much of the 2000s. It was one sign that the U.S. education system was working, perhaps not spectacularly, but at least enough to help struggling students keep pace with the gains of the most privileged and successful.

Today, the country’s lowest-scoring students are in free fall.

The reason is not just the pandemic. For at least a decade, starting around 2013, students in the bottom quartile have been losing ground on the National Assessment of Educational Progress, a key exam that tests a national sample of fourth and eighth grade students in math and reading.

The bottom quartile is made up of students from various backgrounds, but it includes a higher proportion of students with disabilities, students learning English and children from poor families. Since the pandemic, their scores have often continued to fall, even as high achieving students stabilize.

“Whatever is happening to the lower performers is still happening,” said Nat Malkus, a senior fellow at the American Enterprise Institute, a right-leaning think tank, who has tracked the trend.

Researchers point to a number of educational and societal changes over the past decade, including a retrenchment in school accountability, the lasting effects of the Great Recession and the rise of smartphones, which has coincided with worsening cognitive abilities even among adults since the early 2010s.

Figuring out what has happened to the lowest performers is critical, not just for their futures, but for the country’s success.

By leaving behind a huge swath of students, the United States is preparing fewer citizens to do the most technical and high-paying jobs, said Jason Dougal, who studies effective school systems at the National Center on Education and the Economy.

That only widens income inequality in the labor market, he said. And it pushes the United States further from top countries on education — places like Singapore, Japan and Ireland — which succeed not just by raising scores for their top performers, but by lifting up their lowest students.

“To get high average performance, you can’t allow a significant portion of your population to be performing at low levels,” Mr. Dougal said.

More students with challenges

Since the early 2010s, the United States has taken in more immigrants, which means more students learning English have entered public schools. Schools are also serving more students with disabilities.

Those demographic shifts could help explain some change in scores. Both groups are more likely to score below their peers on standardized tests. But it is most likely not the biggest factor, experts said.

The increases are small as a share of the total public school population.

And since 2013, almost every student category has seen significant declines among low performers, said Chad Aldeman, an education researcher and columnist for The 74, a nonprofit news site, who has written about the phenomenon.

The declines have sometimes been greater for more advantaged groups.

For example, in eighth grade math, the bottom 10 percent of proficient English speakers lost more ground than the lowest-scoring English learners, Mr. Aldeman found. Similarly, his analysis showed that the lowest-scoring students who did not have a disability fell more than the lowest-scoring students who did. The bottom scoring middle- and higher-income students lost more ground than the bottom low-income students.

This suggests that there is something about being a low-achieving student, regardless of background, that is driving the trend.

School accountability has faded

One possible explanation is the end of No Child Left Behind, the contentious school accountability law President George W. Bush signed in 2002.

The law is perhaps best known for its legacy of standardized testing, including annual exams in math and reading in third through eighth grade.

But it also put a sharp focus on low performers, part of Mr. Bush’s campaign against what he called “the soft bigotry of low expectations” in public schools. The law set a goal of having all students reach proficiency. Schools were required to break out testing data by race, income and special education status, and schools that did not show progress could face penalties.

It corresponded with a period of rapid improvement in test scores, particularly in math. Reading scores also improved, though more modestly.

The biggest increases were for students at the bottom.

But the law was also deeply unpopular on the left and the right.

Critics argued it was too punitive, with unrealistic goals. Many said it led a “drill and kill” culture of teaching to the test, leaving less time for other important subjects like social studies and the arts.

By the early 2010s, states had gotten waivers from the law, and in 2015, the Every Student Succeeds Act returned power to the states, which in many cases led to more relaxed accountability.

Around the same time, scores among low performers began to fall.

“When we had meaningful accountability at the state and local level, kids were doing better,” said Margaret Spellings, Mr. Bush’s education secretary from 2005 to 2009. “When we stopped doing that, we went the wrong direction.”

Other theories: screen time and the Great Recession

School policies are most likely only part of the picture.

Adults have also been struggling with literacy since 2012, not only in the United States but also in other countries, according to an international survey of 16- to 65-year-olds.

The declines were driven by adults in the bottom tier of literacy, a shift that could not be explained by demographic trends, said Andreas Schleicher, director for education and skills at the Organization for Economic Co-operation and Development, which manages the survey.

He and other researchers pointed to another possibility: the rise of smartphones, which before 2013 had not reached half of American adults. Today, 90 percent of U.S. adults and a similar share of teenagers own a smartphone, as do one in three 9-year-olds.

It’s not entirely clear why smartphone use would have a greater effect on low performers. But smartphones also take time away from other activities. Children (and adults) are reading fewer books than in the past, with low-scoring students being the least likely to read recreationally.

Still, other societal changes could also be at play.

After the Great Recession, states cut school spending, leading to teacher layoffs and other cutbacks. The spending cuts took place over several years, peaking in the 2011-2012 school year. Experts say the cuts were more likely to affect low-scoring students, who tend to be in poor school districts that relied heavily on state funding.

“Many things can be true at the same time, but I’m confident that changes in school spending over time are a big part of it,” said Kirabo Jackson, an economist at Northwestern University, whose research found that students most exposed to Great Recession cuts experienced greater declines in test scores and college attendance.

What now?

Part of the answer may lie in simply focusing on students at the bottom, said Carey Wright, the former state superintendent in Mississippi, where the lowest-performing students have defied national trends.

Mississippi’s lowest-scoring fourth graders have improved since 2013, and eighth graders have fallen less than the national average. Mississippi received widespread attention for dramatically improving reading scores after adopting a new, phonics-based approach to teaching reading in 2013.

But the state also approved a new school accountability policy that same year. Schools receive A-F letter grades based on how well students perform on tests, with an emphasis on the progress made by the lowest 25 percent of students. Literacy coaches are also assigned to the lowest-performing schools.

“We really started drawing teachers’ eyes, principals’ eyes, to who is in the bottom? What do they need?” said Dr. Wright, now the superintendent in Maryland.

Soon, though, there could be even less reliable information on how the lowest-performing students are doing, as the Trump administration seeks to shrink the role of the federal government in education.

As part of a major downsizing at the U.S. Department of Education last month, the Trump administration laid off nearly all federal employees who work on education research, including the National Assessment of Educational Progress, the only test that makes it possible to compare students across the country.

The cuts could hamper the national test, which is required by law every two years.

“Eventually, I hope we’re going to be closing these gaps,” said Thomas Kane, an economist at Harvard University who focuses on student achievement. But the test results are “the only way we’re going to know it.”