One of the arguments that has been used to justify the shortchanging of D.C. charter schools for public dollars is that they have the advantage of being able to draw on more private funds. The findings of a new study shatter that defense and underscore the need for the District to come up with a funding scheme that ensures equity for all of its public school students.

A report released by the Walton Family Foundation, which is pro-charter, showed that charter schools receive significantly less money per student than the traditional system’s schools. The analysis, examining federal, local and state tax dollars as well as private support from foundations, showed $16,361 spent per charter school student in fiscal 2011 compared to $29,145 spent per traditional school student. System schools do have higher costs because of special education, but those costs can’t account for the $13,000 disparity — which, The Post’s Lyndsey Layton reported, is the nation’s largest.

D.C. law mandates equity between the two sectors, but charter school advocates have long complained that the Uniform Per Pupil Spending Formula is implemented in a way that allows the city to funnel more capital and operating funds to the legacy school system. A study last year by Mary Levy, a longtime analyst of public education in the District, found that charters were underfunded by about $1,500 to $2,500 per student in operating funds and $3,000 per student in facilities funds. Most unfair has been the District’s reluctance, if not refusal, to make shuttered schools available to charters; the difficulty KIPP DC is experiencing in finding new facilities to accommodate its top-performing high school is evidence of the District’s indifference. (Washington Post Co. Chairman Donald E. Graham is a member of the KIPP board).

Officials long ago acknowledged the problem of inequity. The D.C. Council in 2010 called for establishment of a study commission that, once formed, couldn’t come up with solutions. Mayor Vincent C. Gray (D), who campaigned on a pledge of equity, has contracted with the Finance Project, a Washington policy research and technical assistance firm, to examine funding policies and come up with specific recommendations, ideally by September. Even more encouraging, he has named Abigail Smith as his deputy mayor for education. Ms. Smith, in charge of school transformation for former chancellor Michelle Rhee and former chair for the top performing E.L. Haynes Public Charter School, understands the need for the city to use its resources in the way that best serves student interests.

Even with less money, charters have outperformed the public school system: Their students score higher on standardized tests and graduate at greater rates. More than 40 percent of public school students attend charters and thousands are on waiting lists. It’s time the District give them their financial due.