After two decades of slow and steady progress in math, U.S. fourth-graders made no improvement over 2007, according to nationwide test scores released Wednesday.

Eighth-graders made headway, posting gains for yet another year.

It is impossible to tell from a single test whether trends are changing. Since 1990, test scores have been rising in both grades, though fourth-graders generally have made bigger gains.

Even so, officials said they were troubled by the lack of progress. Education Secretary Arne Duncan said the results are a call to action.

"None of us should be satisfied," Duncan said in a statement. "We need reforms that will accelerate student achievement. Our students need to graduate high school ready to succeed in college and the workplace."

The results are from the National Assessment of Educational Progress, or NAEP, a series of federally funded achievement tests often referred to as the nation's report card. Kids are tested in nine subjects, but they are tested most often in math and reading. Students generally have been making more progress in math than in reading.

This year's math tests were given to 168,800 fourth-graders and 161,700 eighth-graders in public and private schools in every state.

On a 500-point scale, fourth-graders on average scored 240, unchanged from two years ago. Eighth-graders on average scored 283, up from 281 two years ago.

Also unchanged were children's achievement levels; only 39 percent of fourth-graders and 34 percent of eighth-graders performed at the proficient level, meaning they show the knowledge and skills they should have at that grade level. Eighth-grade scores were up from 32 percent, but that was not statistically different.

Tom Loveless, an education expert at the Brookings Institution think tank, said the results really weren't much different from the 2007 results. It would take another four to six years to see if fourth-grade progress has truly stalled, he said.

"Each of these is kind of like a public opinion poll; it's an estimate," Loveless said. "I think people rush to take each release of test scores far too seriously and try to explain every little wiggle in the data."

Loveless said it is impossible to explain exactly why fourth-grade scores did not budge. "Scientifically, you cannot explain in education why a phenomenon did not happen," he said.

According to the results:

— Just four states and the District of Columbia managed to show improvement in both fourth and eighth grades. The states are Nevada, New Hampshire, Rhode Island and Vermont. The District of Columbia was the only place where kids' scores improved across every group by race, gender and family income.

— Three states saw improvement in fourth grade only; they are Colorado, Kentucky and Maryland. Ten states saw improvement in eighth grade only; they are Connecticut, Georgia, Hawaii, Idaho, Missouri, Montana, New Jersey, South Dakota, Utah and Washington.

— In four states, scores actually dropped among fourth-graders; they are Delaware, Indiana, West Virginia and Wyoming.

In addition, there was no progress from 2007 to 2009 in closing the gap between minority and white students in either grade, though the gap has narrowed somewhat since the 1990s. Black and Hispanic students did make gains at eighth grade, but the gap persisted because white students improved, too.

Experts say this divide, considered one of the toughest challenges in education, is driven by deeply rooted factors. More minority children live in poverty, which is linked to an array of problems that interfere with learning.

Another reason the gap has persisted is demographics — white children made up about 75 percent of students tested in the 1990s but today make up less than 60 percent.

Private school students continue to outperform those in public schools, according to the scores. Private school math scores were 7 points better in fourth grade and 14 points better in eighth grade.

Internationally, U.S. fourth- and eighth-graders have kept improving in math and have gained on some of their toughest competitors. But the most recent tests were done in 2007 and won't be administered again until 2011.