WASHINGTON – Students are doing better on state reading and math tests since the No Child Left Behind Act was enacted five years ago, according to a report Tuesday.

Students made the most progress on elementary-school math tests, according to the report by the Center on Education Policy, a national nonprofit policy group.

The report focused on states where trend data are available. Some states have changed tests in recent years, making it impossible to compare year-to-year results.

Moderate to large gains were found in 37 of the 41 states with trend data on the percentage of kids hitting the proficient mark on elementary-school math tests. None of the states showed comparable declines.

A goal of the No Child Left Behind law is for all kids to be proficient in reading and math, or working on grade level, by 2014.

Another goal is to narrow achievement gaps between children from low-income families and wealthier ones and between minorities and white students. The new report found achievement gaps have narrowed since the law was passed.

Specifically, the study found in 14 of 38 states with relevant trend data, gaps narrowed on the reading tests between black and white students at the elementary and secondary levels. No state reported a comparable widening of the gap.

In math, a dozen states showed a narrowing of the racial achievement gap at the elementary and secondary grade levels. Only Washington state showed a widening of that gap.

Results were generally similar for Hispanic and low-income groups, according to the report.

Just 13 states had enough data to examine whether the pace at which students improved has quickened since No Child Left Behind was enacted.

In nine of those states students improved at a greater rate after 2002 than before: Kansas, Kentucky, Louisiana, New Hampshire, New Jersey, New York, Pennsylvania, Washington and Wyoming.

In the other four states _ Delaware, Massachusetts, Oregon and Virginia _ gains were greater before 2002 than afterward. One possible explanation is that more students, such as those with disabilities or immigrants, were included in NCLB-era tests but not in the earlier ones, according to the researchers.

Education Secretary Margaret Spellings said the study shows No Child Left Behind is working, but the report itself doesn’t assign credit to the law for the improvements made. It states that other state and local initiatives have taken place during the same period that might deserve some of the credit.

“You can’t tease out the effects of any one of the reform efforts, because they all overlap on one another,” said Jack Jennings, president of the Center on Education Policy.

Ross Wiener, vice president for program and policy at Education Trust, a group that advocates for poor and minority children, said he saw good news in the study. “Those trends are encouraging. There’s something to celebrate that’s going on in our schools,” he said.

The rigor of tests varies from state to state, according to Bruce Fuller, a professor of education and public policy at the University of California at Berkeley.

He said states generally set the proficiency bar low, since schools face tough consequences – such as having to fire teachers or administrators – if their students do poorly on the tests.

But Jennings said California, Massachusetts and Florida are examples of states with high standards.

Jennings and Fuller agreed some of the gains may reflect what teachers are focusing on in their classrooms.

“The teachers teach to the test, and that’s a rational response by classroom teachers under pressure to raise scores,” Fuller said.