For years, advocates for poor and minority students have been saying that districts haven’t been doing a good of ensuring that high-poverty schools get their fair share of state and local dollars, including those schools that get federal Title I money to help educate the disadvantaged.

Now, thanks to a wide-ranging study by the U.S. Department of Education, those advocates have some new, very expansive data to back up their point.

Nearly half of all high-poverty schools, including schools that get Title I money, were at least 10 percent below the average school in their district in terms of state and local aid, according to the department’s study, which looked at more than 13,000 districts across the country.

What’s more, the report found inequities in districts that have both Title I schools and non-Title I schools when it comes to personnel costs. More than 40 percent of Title I schools spent less on personnel per student than non-Title I schools in the same district that served the same grades. Check out the full report here.

This is a big deal, said U.S. Secretary of Education Arne Duncan on a conference call with reporters today.

The “findings confirm an unfortunate reality in our nation’s education system,” Duncan said. “Many schools serving low-income children aren’t getting their fair share of funding.” And he added that “in far too many places Title I dollars are filling budget gaps rather than being extra.”

This is a very complicated issue, but it comes down to what’s called the “comparability loophole.” (Cliffnotes version here.) In a nutshell, the Elementary and Secondary Education Act requires that districts give high-poverty schools the same share of state and local dollars as their other schools before the district can tap Title I dollars for disadvantaged students.

The idea is to make sure that Title I money, which is supposed to help districts cope with the added cost of educating poor students, is not just going to fill holes.

So how do districts get around this? The law says that districts don’t have to take actual teachers’ salaries into account. They just have to make sure that teachers at every school in the district are on the same salary schedule. But that means a Title I school with lots of inexperienced teachers (who tend to be cheaper because they’re lower on the salary schedule) could get less state and local money overall than a non-Title I school in the same district.

The department seems to think fixing the problem would be relatively painless for most districts. It issued a policy brief along with the report, which suggested that it would cost districts about 1 percent of their total spending to make up the difference.

Duncan singled out a few districts he thinks are doing a good job making sure high-poverty schools aren’t shortchanged, including Broad prize winner Charlotte-Mecklenberg, in North Carolina.

And there’s been a push in Congress to try to address this issue. U.S. Rep. Chaka Fattah, D-Pa., has a bill on it, and so do Sens. Michael Bennet, D-Colo., and Thad Cochran, R-Miss. In fact, the Bennet-Cochran language was included in a bill reauthorizing the Elementary and Secondary Education Act (aka NCLB) that the Senate education committee passed in October.

Duncan highlighted that provision on the call, commending Sens. Tom Harkin, D-Iowa, and Michael B. Enzi, R-Wyo., the bill’s cosponsors, for their “courage” in including the provision. (That’s one of the few nice things the secretary has said about the substance of the senators’ legislation).

Harkin and Bennet each put out statements today saying that the report’s findings underscore the need to keep the comparability language in the bill.

Here’s Harkin: “This report confirms that the problem of poor kids getting less than their fair share of state and local resources is real in many places.”

Here’s Bennet: “This report is further evidence that the ‘comparability loophole’ is a critical problem that often results in low-income schools subsidizing their high-income counterparts. This is a solvable problem, and closing this loophole is a common-sense step we can take.”

But the comparability language isn’t a slam-dunk. Some folks worry that closing the comparability loophole could lead to forced teacher transfers (there’s language in the Harkin-Enzi bill saying that’s not the intention). And others worry that it could lead to some needless bureaucratic maneuvering as districts try to balance the books. It’s a good sleeper issue to watch in the debate over reauthorization.

So how does the department have all this data? There were new reporting requirements in the American Recovery and Reinvestment Act calling for districts to supply information on a range of previously unreported areas, including school-level expenditures.

And you true-blue data nerds out there are in for a treat. The department has put a searchable database on its website so you can see how schools in your district stack up.