Enactment of the No Child Left Behind law in 2001 followed a decade dominated by a standards and accountability movement that brought deep changes to public schools. Educators and policy makers, in nearly every state, laid out standards as to what students were expected to know in each grade and subject, and required schools to use those standards to guide instruction. They also established standardized testing regimes intended to measure whether students were meeting the standards and to hold schools accountable for student achievement.

The federal law, proposed by President George W. Bush and passed by bipartisan majorities in Congress in his first year in office, sought to build on the standards movement with many new federal rules, including a requirement that states administer reading and math tests every year to students in grades three through eight and once in high school.

It required schools to publish their scores on state tests not just as averages, but broken down by students’ race, sex and other groups, a rule that most educators agree has focused attention on narrowing achievement gaps.

And it raised the importance of the National Assessment, requiring the Department of Education to increase the frequency of its administration in math and reading to once every two years, to help Americans monitor progress toward the goals of universal proficiency and the elimination of the achievement gap.

The latest results on the National Assessment show that in the six years since the law took effect, fourth-grade scores have risen by five points, to 240 from 235. That is slower growth than during the seven years preceding the federal law, when average fourth-grade math scores grew by 11 points, to 235 in 2003 from 224 in 1996.

“Either the standards movement has played out, or the No Child law failed to build on its momentum,” said Mark Schneider, who from 2005 to 2008 was commissioner of the arm of the Department of Education that oversees the National Assessment. “Whatever momentum we had, however, is gone.”

No one can say for certain why achievement progress has slowed since the federal law took effect, and Mr. Schneider and other experts warned that economic, demographic and social factors other than the law itself may be to blame.