Students performed marginally better over the last two years on the nation's most reliable math and reading exam, according to results released Tuesday. But scores are still low, and achievement gaps between students of differing race and incomes remain wide.

On the 2011 National Assessment of Educational Progress, administered every two years, average scores in fourth and eighth-grade math increased slightly, gaining one point each on a 500-point scale since 2009 and continuing a trend of minimal increases since 2003. Fourth-grade reading scores remained stagnant, staying the same since 2007, and eighth-grade reading scores increased by one point since 2009 on the 500-point scale.

"It is not an enormous increase substantively in terms of what students can do," said Jack Buckley, commissioner of the National Center for Education Statistics, the U.S. Department of Education arm that administers the exams. "They're a little bit more likely to get certain questions right."

This year's math scores are the highest to date since 1990, when the test was first administered. But the scores remain dismal: Only 40 percent of fourth graders and 35 percent of eighth-grade students are performing at or above a level defined as "proficient." In reading, despite average scores reaching a similar peak, only 34 percent of students in both grades were rated "proficient."

"The modest increases in NAEP scores are reason for concern as much as optimism," U.S. Secretary of Education Arne Duncan said in a statement. "While student achievement is up since 2009 in both grades in mathematics and in 8th grade reading, it's clear that achievement is not accelerating fast enough to compete in the knowledge economy of the 21st century."

Duncan's sentiment reflects a similar concern among education stakeholders: Though the long-term trends point upward, getting to the stage where all students have a grade-level grasp of reading and math would take years at this rate.

"These results continue a trend of progress both in raising achievements and closing gaps," said Daria Hall, K-12 policy director for the think tank Education Trust. "The progress is far, far too slow."

"We ought to as a country acknowledge what these results mean in terms of the hard work of educators, but not for a second think we can take our foot off the gas."

The exam found consistently large gaps in the performance of white, black and Hispanic students, despite their gradual narrowing over time. For example, in fourth-grade math, 9 percent of white students performed at or above the highest level, compared to only 2 percent of Hispanic students and 1 percent of black students. A greater number of students than in 2009 were found to be eligible for free- or reduced-price lunch, a common measure of poverty in education.

"It's more of the same: It's good that the long-term improvement trend is being shared among different economic groups and racial-ethnic groups, but we're not seeing closing in gaps," said Kevin Carey, policy director of the D.C.-based think tank Education Sector. "There have been other time periods where we saw rapid gap closing, in the 1970s in particular. We haven't been in that for a long time."

One bright spot may be that the scores come on the horizon of major changes to federal education policy.

The No Child Left Behind Act, passed a decade ago under George W. Bush, mandated the regular testing of students in order to hold states accountable for student performance. It also sanctioned schools if they did not meet performance benchmarks known as "Adequate Yearly Progress." But these benchmarks may be on their way out, whether Congress reauthorizes No Child Left Behind, or the Obama administration grants waivers to states that would exempt them from meeting the yearly benchmarks.

Depending on which reform strategy wins, new changes may also be introduced that mandate the grading of teachers based on students' test scores instead of experience. Further, 44 states are in the process of implementing a common set of standards for K-12 reading and math, known as the Common Core, that are aimed at making students college ready.

"We're seeing slow but steady progress, but nothing to write home about," said Michael Petrilli, executive vice president of the right-leaning Thomas B. Fordham Foundation. "If we're going to try to bump it up to the next level, we're going to need new interventions. Maybe those could be in teacher quality, digital learning, or the Common Core."

Carey agreed. "Nobody could say continuing to do the same thing will produce any kind of significant new improvement," he said.

The NAEP data is widely considered the most reliable in education. "NAEP is all we have," Carey said. "There are no other numbers that are credible for making judgments about the long-term progress of the nation's students."

In math, NAEP tested 209,000 fourth graders and 175,000 eighth graders, covering both private and public school students chosen to be nationally representative. In reading, 213,000 public and private school fourth graders and 169,200 eighth graders took the exam.

Results are broken down into three levels of achievement: "basic," which, according to NCES, "denotes partial mastery of the skills and knowledge" needed for proficiency; "proficient," which "represents solid academic performance" and "advanced," which "represents superior work."

The reading exam aimed to assess students on comprehension in both literary and informational texts. Fourth grade reading proficiency denotes the ability "to integrate and interpret texts and apply their understanding to draw conclusions and make evaluations." In eighth grade, proficiency is intended to signal the ability to summarize ideas, "make and support inferences," and "connect parts of a text and analyze features." The math exam tests students across five categories, including algebra and geometry.

Average fourth-grade scores were higher this year than in 2009 in Alabama, Hawaii, Maryland and Massachusetts, but were lower in Missouri and South Dakota. Average eighth-grade scores did not decrease in any states, and were higher on average in Hawaii and Maryland. In math, fourth-grade scores decreased in New York and eighth-grade scores lost ground in Missouri. In reading, fourth-grade scores slipped in Missouri and South Dakota.