Bad news for America's schools: Student achievement in math and reading is on the decline, according to National Assessment of Education Progress scores released Wednesday.

The National Assessment of Education Progress, or NAEP -- called the Nation's Report Card -- is an exam given to fourth-grade and eighth-grade students throughout the country by the National Center for Education Statistics, a branch of the U.S. Department of Education. While students' scores have increased overall since the 1990s, results released Wednesday show a slight decline between 2013 and 2015.

NAEP is the "largest continuing and nationally representative assessment" of America's students, and has been referred to as the "gold standard" of student assessment.

Only 36 percent of fourth-grade students and 34 percent of eighth-grade students in 2015 scored high enough to be considered proficient or above in reading. In math, 40 percent of fourth-grade students and 33 percent of eighth-grade students scored proficient or above.

While the 2015 results are disappointing, experts caution that they should not yet be interpreted as a downward trend.

"One downturn does not a trend make, and thats what were comfortable in saying about the data," Peggy Carr, acting commissioner of NCES, said on a call with reporters. "Were trying not to read too much into a decline at this point."

Carr also said the drop in scores was "an unexpected one. ... This isnt a pattern that we saw coming."

For the most part, states' average scores stayed the same or declined in 2015. In only a few places -- including Mississippi and Department of Defense schools -- did student scores increase. Achievement gaps between white and minority students did not change significantly in 2015 nationwide.