Roughly two-thirds of elementary schools surveyed by the nonpartisan Center on Education Policy reported increasing math and reading time since the law was passed in 2001.
The law requires annual testing in reading and math in grades three through eight and once in high school. Schools face sanctions if they miss testing benchmarks.
"Clearly what this is showing is, what schools are held accountable for is what they put the emphasis on," said Jack Jennings, president of the Washington-based center.
The report, being released Wednesday, says that of the districts reporting an increase, elementary schools are spending on average 37 minutes more per day on reading, math or both since the law was passed.
Nearly half of the districts said they have cut time in elementary schools for non-tested subjects such as social studies, science, art, music and gym. The cuts across these various subjects totaled about 30 minutes a day, according to the report.
About a quarter of middle schools reported increasing time spent on reading or English. One in five said they increased time spent on math. They didn't report cuts in other subjects.
In some cases, schools appear to be adding math and reading time to lessons in other subjects, meaning they might be teaching both reading and history at the same time, Jennings said.
Education Secretary Margaret Spellings, in response to the study, said: "If children can't read, they can't learn history. Before No Child Left Behind, little was done to hold schools accountable for teaching our children basic, critical skills."
In the Tigard-Tualatin school district, outside Portland, Ore., things like assemblies and special projects have been cut, said Susan Stark Haydon, the district's director of community relations.
"Being able to read is key. If you can't read, you have very little chance of being successful in life," Haydon said. But she added, "I think that it's too bad that some of the things that made school fun aren't there anymore."
The latter is a sentiment echoed by many No Child Left Behind critics, including teachers unions, lawmakers and several of the current presidential candidates. The report out Wednesday is likely to add to the debate over how the law is influencing classroom practices.
The report found at the high school level, students have been taking more math and science coursework, which may be driven by state graduation requirements.
The report doesn't address whether that added time is coming at the expense of other subjects. However, A separate federal study found declines in time spent on vocational education among high schoolers.
A recent Education Department study of first through fourth grades showed that between 1999 and 2003 more time was spent on reading. The study showed a slight drop during that period in time spent on math, science and social studies.
The report out Wednesday, however, is based on more recent data. The surveys were all conducted during the 2006-2007 academic year.
Schools are facing tougher consequences under the No Child Left Behind law, which could explain the recent spikes in time spent on math and reading in the new report.
It showed that districts with at least one school identified as needing improvement under the education law were more likely to add additional math and reading time to the school day than other districts.
"It shows that the stronger the threat of sanctions, the more the curriculum narrows," said Brian Stecher, a senior social scientist at the Rand Corp.