Washington ? The No Child Left Behind law was supposed to level the playing field, promising students an equal education no matter where they live or their background. From state to state, however, huge differences remain in what students are expected to know and learn.

Each state sets its own standards for subjects such as reading and math, then tests to see whether students meet those benchmarks. It’s a practice under increasing scrutiny as Congress prepares to review the five-year-old law.

“Fourth-grade kids in the District of Columbia are learning different math from kids across the (Potomac) river in Virginia. It’s crazy. Math is math,” said Michael Petrilli, vice president for policy at the Thomas Fordham Foundation, a Washington-based education reform group.

The solution, say Petrilli and other advocates, means standards of learning that are uniform nationwide.

Republicans generally have opposed national standards. GOP lawmakers say state and local officials know what is best for their students and, as the primary funders of elementary and secondary education, should have primary say in running schools.

Many Democrats, along with education reform and business groups, say a patchwork of standards is inefficient. They also say students in states with low standards will have trouble competing in the global economy. Many other industrial nations have more stringent standards than those in the U.S.

There are signs states are wrestling with the problem. Some are talking about sharing tests and looking at benchmarks that would identify the skills U.S. students should have when they finish high school.

Advocates of national standards say the No Child Left Behind law is encouraging states to set low standards so schools can avoid consequences that come with missing annual progress goals.

Schools that miss those targets must take steps such as paying for tutoring or overhauling staffs. All students have to be proficient, which generally means working at grade level, in reading and math by 2014.

At least one state, Missouri, lowered its standards after the federal law went into effect.

Supporters of national standards point to the vast differences between student performance on state tests compared with a rigorous national one as evidence states are using weak standards.

Massachusetts Democratic Sen. Edward Kennedy, chairman of the committee overseeing education issues, has proposed legislation generally encouraging states to raise their standards to a consistent level, as has Sen. Chris Dodd, D-Conn.

Dodd’s legislation won the endorsement of the National Education Association, the largest teacher’s union.

Among educators, there is a concern national standards would become outdated and that changing them would be difficult and bureaucratic.

Brenda Dietrich, a superintendent in the Topeka area, said she has not formed an opinion on national standards, but does see a logic to them.

“If we’re all going to be held to a standard, it certainly would be nice if it were the same standard,” Dietrich said.