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.