Secretary of Education Arne Duncan, right, and John King, who will succeed Duncan as secretary in December, visited a D.C. school this month. (Evelyn Hockstein/For The Washington Post)

Fourth-graders and eighth-graders across the United States lost ground on national mathematics tests this year, the first declines in scores since the federal government began administering the exams in 1990.

Reading performance also was sobering: Eighth-grade scores dropped, according to results released Wednesday, while fourth-grade performance was stagnant compared with 2013, the last time students took the test.

And the tests again show large achievement gaps between the nation’s white and minority students as well as between poor and affluent children, an indication that the nation’s disadvantaged students are not gaining ground despite more than a decade of federal law designed to boost their achievement.

Researchers have long cautioned that it is difficult to identify the cause of any fluctuation in scores on this testing program, the National Assessment of Educational Progress (NAEP), which is also known as the Nation’s Report Card. But many people look to NAEP scores as an important barometer of U.S. student achievement because they are the only exams that have been given nationwide over a long period of time, capturing the performance of rich and poor children of all ethnicities in urban, suburban and rural communities.

The year’s declines come amid a period of great tumult in American public education.

Recent demographic shifts mean that schools are grappling with the challenge of educating an increasing number of students who come from low-income families and are learning how to speak English. And in recent years, most states have adopted sweeping educational policy changes, including teacher evaluations tied to test scores and Common Core academic standards that have changed what and how students learn in the classroom.

U.S. Education Secretary Arne Duncan defended those policies in a call with reporters Tuesday, saying that massive changes in schools often lead to a temporary drop in test scores while teachers and students adjust. But the new standards and other policies, Duncan said, are poised to improve student achievement — and students’ lives — in the long terdatam.

“Big change never happens overnight,” Duncan said. “I’m confident that over the next decade, if we stay committed to this change, we will see historic improvements.”

Randi Weingarten, president of the American Federation of Teachers, said that slipping NAEP scores are evidence that the nation’s focus on using standardized tests to judge teachers and schools has failed. The scores should trigger a change of course, she said, pointing to the Obama administration’s acknowledgment that students are spending too much time taking standardized tests of dubious value.

“Not only is there plenty of anecdotal evidence that our kids have suffered, these latest NAEP scores again show that the strategy of testing and sanctioning, coupled with austerity, does not work,” Weingarten said in a statement.

Students at U.S. public and private schools have taken the NAEP every two years since the early 1990s. The exam is the country’s most consistent measure of K-12 progress, and because it has been in place for so long, it can offer insight into the effects of demographic and policy changes.

But researchers caution that deeper analysis is needed to understand the potential causes of this year’s drop. And they said it’s too soon to tell whether the results are the beginning of a trend or just a blip.

The new results — on a scale of 0 to 500 — show two-point losses in eighth-grade math and reading and a one-point drop in fourth-grade math. Fourth-grade reading scores were statistically unchanged.

Scores have risen considerably since the first exams in the 1990s and, despite this year’s declines, are still among the highest posted by American students.

The 2015 scores show that 64 percent of fourth-graders and 66 percent of eighth-graders are not considered proficient in reading. In math, 60 percent of fourth-graders and 67 percent of eighth-graders are not considered proficient.

The new data also show how states and 21 large cities fared. Individual state performance mirrored the nation’s, with more states showing scores that dropped rather than increased. The news from cities was somewhat more positive: On average, performance among urban school systems was flat compared with 2013.

Maryland had some of the largest drops in the nation and was the only state to see declines in both subjects at both grade levels. They came as Maryland tested many more students with disabilities and non-native English speakers than before, bringing it in line with other states.

The District bucked the national trend, posting some of the biggest increases in the country at the fourth-grade level. Eighth-grade scores were flat. It was the second time in a row that the city, whose school system long had a reputation as one of the nation’s most troubled, has stood out for its improvement on the NAEP.

D.C. Schools Chancellor Kaya Henderson attributed the success in part to the city’s early adoption of the Common Core State Standards. She also credited consistency of leadership — Henderson is in her sixth year as chancellor — and the city’s investment in universal preschool for 3- and 4-year-olds.

Virginia’s scores were virtually unchanged from 2013.

The highest-scoring state in three of the four tests was Massachusetts, which has often topped NAEP rankings. Minnesota, New Hampshire and Vermont also were top performers in 2015. New Mexico and Mississippi were among the lowest-scoring.

Matthew Chingos, a researcher at the Urban Institute, said it is not very useful to compare overall state scores to one another because states are educating such different populations of students. A state with a more challenging student population can be doing a relatively good job with those students but still trail states with student populations that are whiter or more affluent.

For example, in 2013, fourth-grade students who were learning English as a second language scored higher in Texas than they did in Oregon, while other students in the two states scored about the same, according to a brief Chingos published Monday.

But Texas has far more non-native English speakers than Oregon, so its overall NAEP score was lower.

“If you want to compare across states, if you want to say how kids in Massachusetts versus Mississippi are doing, you really do need to make these adjustments,” said Chingos.

“It can have a big impact on which states are doing better and which are doing worse.”