Kindergarten student Javier Velsaco Lemus (center), sings in music class at Guilford Elementary school, February 6, 2015. The school system in Loudoun County, is one of the wealthiest counties in the U.S. by some measures. (Evelyn Hockstein/For The Washington Post)

The gap in math performance between poor students and their wealthier peers is due in large part to the systemically weaker math content in schools that teach low-income students, according to a new study released Wednesday.

In a peer-reviewed study published in the journal of the American Educational Research Association, researchers analyzed test scores of students who took the 2012 Programme for International Student Assessment, an international test given by the Organisation for Economic Co-operation and Development (OECD).

They found that a large amount of the difference in math scores between poor and wealthier students can be traced to unequal access to strong math content in school. The researchers said the problem is worldwide and not isolated to the U.S.

But they estimated that nearly 40 percent of the gap in U.S. student performance in math can be traced to that unequal access; the researchers attributed the remaining 60 percent to family and community background.

“We’ve had this debate for some time about whether schooling can help at least lessen the impact of social class inequality,” said William H. Schmidt, one of the authors of the study. “That’s part of the American Dream — if children come, take school seriously and work hard, they have a chance a better existence. But we find that the schools are making things worse, not helping.”

Schmidt, Nathan Burroughs, and Richard Houang, all of Michigan State University, and Pablo Zoido, of OECD, analyzed the test scores of 300,000 students, all either 15 or 16.

In nearly all the countries that gave the test, a significant amount of the difference in scores between low and high income students could be traced to weaker math content in the schools that teach low-income students.

And although “tracking” — teaching different groups of students different material — has been frowned upon in recent years by most U.S. public schools, the practice endures in a repackaged format, Schmidt said.

“There’s a certain amount of tracking that still goes on, ” Schmidt said. “A lot of it is what I call shell games. If you look at transcripts, you’ll see a school offers 10 different kinds of Algebra classes — Algebra 1, Algebra A, B and C and so on. And a parent thinks, ‘Oh, my kid is doing fine, he’s taking Algebra.’ But upon closer examination, that student is getting something different. And it’s showing up in our analysis quite strongly.”

The study comes as record numbers of homeless and poor children are filling classrooms in U.S. public schools.

On average, across the 33 OECD countries studied, roughly one-third of the difference in math test scores between poor and wealthier students was due to unequal access to challenging math in school. But that factor varied across countries, from a low of 10 percent in Iceland and Sweden to almost 60 percent in the Netherlands, according to the study.

“One thing people say is ‘Oh, you can’t make access more equal, social class is always related to what you’re getting in school,’” Schmidt said. “But look at Sweden, where the social class inequity is greater than in the U.S. But all the kids get the same basic opportunities, the same content coverage. They’ve taken (the inequity) out of the schools.”

One solution may be the Common Core State Standards, a uniform set of K-12 math and reading academic standards rolled out by more than 40 states and the District of Columbia in the years since the PISA test that formed the basis for the new analysis, Schmidt said.

But the key is how the standards are applied to the classroom, and whether all students get access to the same content, particularly in the middle school and high school years, when grouping by “ability” is most prevalent, he said.

“Almost 40 percent of social class inequality is coming through schooling,” he said. “If we can just figure out policies that can eliminate that, we can lessen (inequality) by 40 percent. That’s one big thing we can do about it.”