The gap between high- and low-achieving students widened on a national math and science exam, a disparity that educators say is another sign that schools need to do more to lift the performance of their most challenged students.

Averages for fourth- and eighth-graders on the National Assessment of Educational Progress, also called the Nation’s Report Card, were mostly unchanged between 2015 and 2017. The exception was eighth-grade reading scores, which rose slightly.

But scores for the bottom 25 percent of students dropped slightly in all but eighth-grade reading. Scores for the top quartile rose slightly in eighth-grade reading and math.

The slippage among the nation’s lowest-performing students raised concerns among educators and experts, who say it should be closely monitored.

“It certainly is an alarm bell,” said Tom Loveless, a nonresident senior fellow who studies education at the Brookings Institution, a think tank. “It’s something to watch and monitor, especially if this persisted.”

Nationally, 37 percent of fourth-graders were considered pro­ficient in reading, and 40 percent reached this threshold in math on the 2017 exam, according to data released Tuesday by the federal government’s National Center for Education Statistics. Thirty-six percent of eighth-graders were considered proficient in reading, and 34 percent in math.

“The report card is in, and the results are clear: We can and we must do better for America’s students,” Education Secretary Betsy DeVos said Tuesday in a statement. “Our nation’s reading and math scores continue to stagnate. More alarmingly, the gap between the highest and lowest performing students is widening, despite billions in federal funding designated specifically to help close it.”

Experts caution against drawing conclusions from a single set of exam results. But the test is considered an important barometer of the health of the nation’s schools and the most consistent measure of student achievement. The tests, first administered in the 1990s, are given to fourth- and eighth-grade students every two years and to high school students less often.

It is not the first national exam to show slipping results among the nation’s lowest-performing students. On an international reading exam given to fourth-graders in 2016, U.S. performance slipped in the rankings, with the steepest decline posted by the bottom 25 percent of students.

The scores add to a worrisome body of evidence that academic performance in the United States is stagnant or slipping. Scores for fourth- and eighth-graders dropped in math in 2015, the first time students in those grades had posted a decline; there were no statistically significant differences in 2017 scores. Eighth-grade reading exam scores also slipped in 2015 but rose in 2017.

“We learned that the decline in scores we saw in 2015 was a real phenomenon, not a temporary blip,” said Martin West, a Harvard University education professor. “The progress that we’ve had in the first decade of this century has entirely stalled — and in fact we’ve lost a bit of ground.”

Peggy G. Carr, associate commissioner for the National Center for Education Statistics, said there were no statistically significant changes when it came to different categories of students. This means black and Hispanic students continue to trail their white counterparts on the exam. Students from low-income households also performed below the national average, as did special-education students, though they posted significant gains in 2017 compared with two years earlier.

“While several states demonstrated progress in improving student achievement and narrowing achievement gaps, it is clear we as a country must do better by all of our students, especially our lowest-performing kids,” said Carissa Moffat Miller, executive director of the Council of Chief State School Officers, which represents state school chiefs.

Last year marked the first time students took the test on computers. Carr said the transition reflects the shift in education, with more students learning and communicating online.

“We are just ecstatic about being able to move these assessment into a digital format,” Carr said.

Research shows digital assessments are tougher for students than paper-and-pencil tests. So, Carr said, her federal center adjusted results so the change in format “would not influence the comparisons and the trends that we are reporting.”

But John White, state education superintendent of Louisiana, said the center did not account for differences in demographics among states. In some states, students are less likely to have access to a home computer. Some students have experience taking digital exams, while other states administer only paper tests.

“You’re giving a kid a tablet, and that student may have never used a computer before to take a test. They may have never used a computer at all,” White said. “By making the same adjustments in all states, you’re not only giving some states not enough credit, you’re giving other states too much credit.”

Average scores for most states reflected the national trend, with most scores remaining unchanged from 2015. Notably, Florida posted gains in fourth-grade math and eighth- grade reading and math.

“Florida has been at the forefront of bold, comprehensive education reform for decades,” DeVos said her statement. “From accountability, to literacy, to teacher certification and recognition, to providing parents more freedom to select the learning environment that best fits their students’ needs, Florida is rethinking education.”