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The National Women’s Law Center analyzed federal data from more than 16,000 high schools, looking at the gap between the overall proportion of girls in each school and the proportion of slots on that school’s sports teams that were filled by girls. In more than a quarter of schools, the gap was larger than 10 percentage points.

“Girls across the country are still not getting equal chances to play sports and that’s a big problem,” said Neena Chaudhry, senior counsel at the National Women’s Law Center, the nonprofit organization that has served as a watchdog for school districts’ compliance with Title IX, the federal civil rights law that prohibits sex discrimination.

The new analysis is based on the U.S. Education Department’s Civil Rights Data Collection for the 2011-12 school year.

Chaudhry acknowledged that some people see gender gaps not as evidence a violation of girls’ civil rights but as evidence that girls just aren’t interested in playing sports. That’s an argument that people have been making since before Title IX was passed, she said, and yet girls’ participation in sports has grown as more opportunities have become available to them.

Schools with big gender gaps should be surveying their female students to find out whether they’re interested in athletics, and if so, what sports they want to play, she said. “A school can’t just sit back and say, ‘Well, girls aren’t interested in playing’ if they haven’t even asked the girls,” Chaudhry said.

Schools can show compliance with Title IX by showing that they don’t have a gap — i.e., that the proportion of girls in their school is roughly equal to the proportion of sports slots allotted to girls. Schools with gaps larger than 10 percent are not necessarily violating Title IX, but Chaudhry said the center has never come across a school with such a large gap that was following the law.

In 2010, the National Women’s Law Center filed Title IX civil rights complaints against a dozen school systems nationwide based in part on a similar analysis of their schools’ gender gaps. Most of those complaints have been resolved, with the school districts entering into settlement agreements that require them to survey their female students and add sports that girls want to play.

Research has shown that girls who play sports are healthier, have higher grades and test scores and better employment outcomes than those who don’t, Chaudhry said. “We want to make sure that girls get a fair shot.”

The center’s new analysis shows that there are high schools with large gender gaps in every state across the country, but they are not evenly distributed.

The states with the largest number of high schools with large gender gaps were all in the South: Louisiana, Mississippi, Georgia, Alabama, Tennessee and South Carolina. In those six states and the District of Columbia, more than half of public high schools had such large gender gaps.

They are all states with high numbers of minority girls, who are especially likely to have poor access to school sports, according to a previous report by the center.

The center filed one of two Title IX complaints against D.C.’s public school system in recent years, alleging that girls had far less opportunity than boys to participate in sports. One of those complaints has been resolved, and D.C. schools officials have said they have made many changes in the last several years in order to ensure equal access to its athletic teams.

The center ranked the District 50th on its state ranking for gender equity in high school sports. Georgia, where two-thirds of schools had large gaps, was 51st. A spokesperson for the state’s education department did not immediately respond to a request for comment but said officials were looking at the report.