Nearly 1,000 school district boundaries separate communities where students are significantly more white and wealthy from communities where students are significantly more black and Latino and poor, a new study shows – a finding that shines a spotlight on the severe racial and economic segregation in U.S. public schools.

"What we take away here is the whiter a community is on one side of the border, or the more nonwhite a community is on the other side of the border, the bigger the difference is in funding for the more disadvantaged," says Rebecca Sibilia, CEO of EdBuild, a nonprofit that focuses on education funding inequality and which published on Thursday its latest report highlighting the fiscal impact of segregation.

The report's release marks the 45th anniversary of the 1973 education equity Supreme Court case, Milliken v Bradley, in which a conflicted bench ruled 5-4 that school districts were not obligated to desegregate unless it had been proven that the lines were drawn with racist intent.

The case, which came nearly two decades after the landmark Brown v Board of Education ruling that separate is not equal, considered the planned desegregation via busing of public school students across district lines among 53 school districts in and around Detroit.

In 1970, the NAACP sued Michigan state officials, arguing that although no official policy of segregation existed, Detroit and surrounding counties had enacted economic and housing policies that locked out black families and increased racial segregation in schools. The district judge agreed, as did the Sixth Circuit Court, but state officials won out in their challenge to the Supreme Court, effectively cementing the segregation that exists between school districts.

"School district lines, however innocently drawn, will surely be perceived as fences to separate the races when, under a Detroit-only decree, white parents withdraw their children from the Detroit city schools and move to the suburbs in order to continue them in all-white schools," the late Justice Thurgood Marshall wrote in a dissenting opinion.

Today, the EdBuild report shows, nearly 9 million students in the U.S., or one in five, live next door to a significantly whiter and richer school district.

In some areas of the country the segregation is more severe than others. Of the 13 school districts that surround Philadelphia, for example, two-thirds are at least 25 percentage points whiter and have at least 10 percent more funding for their schools than the city schools – equating to an average of $5,000 or more in per-pupil funding.

One of those districts, Lower Merion, is 60% whiter than Philadelphia and students there receive $30,000 more per student. In Philadelphia, 86% of students are non-white.

"The very fractured nature of our school system, combined with the federal government's inability to step in to equalize things for kids, is putting so much power and weight on this school district border that the border itself is what's fundamentally preventing kids from accessing opportunity that's across the street," Sibilia says.

Notably, the EdBuild report comes on the heels of the first Democratic primary debate, when Sen. Kamala Harris of California pressed former vice president Joe Biden about his opposition to busing as a form of school desegregation in the 1970s when he served in the U.S. Senate.

"You also worked with [segregationist senators] to oppose busing," Harris said to Biden during the debate. "And there was a little girl in California who was part of the second class to integrate her public schools and she was bused to school every day. And that little girl was me."

"I did not oppose busing in America," responded Biden, who served in the U.S. Senate representing Delaware from 1973 to 2009. "What I opposed is busing ordered by the Department of Education. That's what I opposed."

The exchange strikes at the heart of the decision in Milliken v Bradley: If separate isn't equal, then whose responsibility is it to ensure schools are integrated?

The issue is expected to resurface again next week when candidates meet for the second debate. Biden, Harris and Sen. Cory Booker of New Jersey, who has also criticized the former vice president for his stance, will all take the stage the same evening.

Civil rights advocates say they're more interested in hearing whether the candidates consider school segregation a problem, whether they'd prioritize it if elected and, if so, how they'd tackle it. But when it comes to K-12 education, the candidates have been more focused on boosting funding for big federal programs like Title I, which supports school districts with lots of poor kids, special education and teacher salaries.

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In fact, during the NAACP's annual convention Wednesday in Detroit, where 10 candidates sat for about 10 minutes for a one-on-one interview, the issue of school segregation rarely surfaced, despite introductory remarks from former New York City mayor Michael Bloomberg, who is not running for president, who urged candidates to do just that.

"For far too long, zip code and skin color have determined a child's education," Bloomberg said. "I believe fixing it must be our top priority for our country, and for our next president because kids in Harlem and Detroit and Memphis are every bit as equal to kids in Beverly Hills and Grosse Pointe and Scarsdale, and they deserve schools and teachers that are every bit as good."

"The sad fact is that the schools doing the worst job preparing students for success are generally in African-American and Latino neighborhoods," he said. "Everyone knows that."

Aside from former HUD Secretary Julian Castro, who spoke at some length about families of color who have been locked out of certain neighborhoods, and Massachusetts Senator Elizabeth Warren, who briefly talked about an affordable housing plan to help communities that have been redlined, none of the other eight candidates referenced segregation.

For those who pay attention to K-12 funding – an issue that's increasingly become more mainstream as states experiment with ways to make their school financing systems more equitable – the discrepancies between poor districts with lots of students of color and rich districts with lots of white students is not a surprise.

School district budgets are intrinsically tied to property taxes, meaning schools in wealthier communities automatically have a larger pot of local funding to begin with. Many states try to account for this when distributing K-12 funding, directing more money, for example, to school districts with lots of poor kids.

But wealthy districts benefit from many other advantages, too, like involved PTAs that can raise hundreds of thousands of dollars, even millions in some communities , to help shoulder costs that would otherwise come out of the district's budget. All this while schools in poor communities are often hampered by compounding issues of economic distress, drug abuse and high crime rates.

"A lot of time people just assume that federal dollars will come to make up the gap," Sibilia says. "What we're trying to show is that even in communities where you take federal funding into the equation, we still aren't getting to an equal place."

To be sure, the Milliken case doesn't restrict school districts and states from pursuing ways do desegregate across school districts, but few have.

In the last year, however, as public debate over K-12 and higher education issues has drifted toward equality and access, a handful of state legislatures are considering serious changes to housing policies and education funding as a way to better integrate schools, among other things.

Texas and Illinois are among a handful of states to recently update the formula used to distribute state education funding to better target the most underserved communities, which indirectly addresses segregation. And a few places are considering housing policies as a way to get to integration: This month Oregon adopted a law that allows duplexes to be built in areas traditionally zoned for single-family homes, so long as the city has more than 10,000 people. The city council in Minneapolis adopted a plan to eliminate single-family zoning altogether, paving the way for duplexes and triplexes to be built anywhere in the city.

Advocates are also watching closely a pair of school segregation lawsuits playing out in real time in Minnesota and New Jersey.