WASHINGTON - The nation's students are getting better at math, but their reading performance is mixed, with slight progress in grade four and a slip backward in grade eight.

The 2005 scores come from the National Assessment of Educational Progress, a federal test considered the best measure of how students in every state perform on core subjects.

Released today, the results will be widely used as a way to measure whether the country's emphasis on math and reading - fueled by President Bush and Congress - is working.

"What we've got here is a pretty satisfactory elementary performance - better math and reading," said Darvin Winick, chairman of the National Assessment Governing Board, the bipartisan panel that oversees the test. "The eighth-grade performance is much more mixed."

The strongest results came in math, particularly in fourth grade, where scores were up for every major racial and ethnic group since the last test in 2003. Math scores increased slightly in eighth grade, where black and Hispanic students narrowed their gap with whites.

Overall in math, 36 percent of fourth-graders could handle challenging material, up from 32 percent in 2003. Among eighth-graders, 30 percent reached at least that "proficient" level, up from 29 percent.

But in reading, another skill vital for success in other subjects, scores weren't so solid.

The average reading score rose one point to 219 on a scale of 500 in fourth grade, a statistically significant increase. But only 31 percent of fourth-graders showed mastery of demanding material - the figure that typically matters the most. That performance was flat compared to 2003.

The same share of eighth-graders, 31 percent, were proficient in reading. That performance actually dropped compared to 2003.

Winick said that taking a longer view, back to 2000, shows steady gains in math in both grades. And reading is up overall in grade four, too. But, he said: "There's no dancing around the flat eighth-grade performance in reading. I think it's a national problem."

In perspective, the numbers show a majority of students don't have the math or reading skills they should based on rigorous federal standards.

Much higher numbers of students in both subjects showed basic skills, a lesser category meaning partial mastery of grade-level work.

As usual, the numbers left much room for interpretation, from the scores themselves to whether the federal government's more aggressive role in education was having an effect.

Education Secretary Margaret Spellings said she was heartened to see narrowing academic gaps between whites and minorities in both grades and both subjects. She said the core principles of the 2002 No Child Left Behind Act, including annual testing in reading and math and reporting of scores for all groups of students, were paying off for younger children.

"Most of the investment has been in grades K-3, where we can get the most bang for the buck, with little kids - getting them on track to be good readers," she said. The president has also proposed extra help for older students, she added, "because we need not give up on middle and high school students who have intractable reading issues."

Patricia Sullivan, director of the independent Center on Education Policy, questioned why gains weren't higher given the time schools devote to reading and math. States choose varying curriculum, which means some students may face unfamiliar material on the test.

Still, she said: "The numbers aren't jumping in big ways, which tells us something's not right here. We're not doing enough." If scores only increase by a point every two years, she said, "Boy, we've got a long way to go."