Math test scores for fourth and eighth grade students across the United States dipped in 2015, marking the first such decline in 25 years, according to a U.S. Department of Education report released on Wednesday.

The National Assessment of Educational Progress, known as the "nation's report card," found the average of test scores for more than half a million students declined.

Test scores for fourth and eighth grade students dropped two points to 240 and three points to 282, respectively, from 2013 to 2015. The test, which is administered every other year, is scored on a scale of 0 to 300.

The report did not offer reasons for the dip and experts say the decline does not span enough time to allow firm conclusions to be drawn.

"At this point we don't know if it's a one-year fluke or something else," said William Mathis, managing director of the National Education Policy Center at the University of Colorado Boulder.

Mathis said that while more data is needed, several factors could have contributed to the decline.

One is the use of rigorous standardized testing initiatives in schools over the past 14 years, which he says can exhaust students and educators.

"There's a lot of drill and practice and just incredibly dull teaching," Mathis said.

The testing has come under scrutiny by educators and politicians, who say the emphasis on standardized testing has overstressed children, teachers and parents.

President Barack Obama this week called for curbs on standardized testing for that same reason.

Reading scores were down three points for eighth graders while fourth-grade levels were unchanged, according to the report.

The economy and inequalities disproportionately affecting children of color are other possible reasons for the decline in test scores, Mathis said.

The assessment is used by educators, policymakers and researchers to assess learning progress and develop ways to improve education, education department officials said.

(Reporting by Laila Kearney; Editing by Eric Walsh)