The Two Rivers Public Charter School at Florida Avenue and Fourth Street NE. (Mark Gail/The Washington Post)

Patricia A. Brantley is chief executive of Friendship Public Charter School. Irene Holtzman is executive director of Friends of Choice in Urban Schools

If you visit a D.C. public park on any given weekend, you will see children of all ages. The younger ones might be on the slides or the swings. You’ll find the older ones riding bikes or playing basketball. Taking in the scene, you would have a hard time guessing where each child goes to school because in the park, they’re all D.C. kids.

When Monday comes, however, the city stops treating its children — and the public schools they attend — equally. The government provides far more financial support to its traditional public schools than to the public charter schools that serve nearly half of D.C. public school students.

It’s been this way for years, and not only here. Advocates for equity in how D.C. schools are funded finally went to court to get the problem fixed. Recently, though, a judge’s disappointing decision let the District government continue to flout its own funding law and shortchange students whose families have chosen public charter schools.

The law, the D.C. School Reform Act of 1995, requires all public schools’ operating costs to be funded exclusively through a uniform formula based on their enrollment. All public schools — district-run and charter — receive the same per-student dollar amount for each child in the same grade or with similar special needs.

The inequities arise in the additional city services that are made available at no cost to campuses overseen by D.C. Public Schools. For instance, the city’s maintenance teams take care of DCPS buildings and grounds. Until recently, city attorneys helped DCPS schools with their legal needs. The city contributes to DCPS teachers’ retirement. Charter schools, on the other hand, bear such costs themselves, reducing what they can spend on teaching and learning.

Multiple studies, including one by the District itself, have called out this inequity. School-finance expert Mary Levy found that DCPS, but not charters, received between $72 million and $127 million annually on top of formula funds from 2008 to 2014, when charter school leaders filed their lawsuit. Thinking back to the playground, imagine neighborhood friends on a seesaw. The DCPS school where one child goes got $2,150 more than the other child’s school. That’s imbalanced — even for a seesaw.

The School Reform Act is clear that simple multiplication is what should determine public funding for every school. Multiply the uniform per-student dollar amount by the number of students enrolled, and that’s what a school should receive. In that calculation, too, the District government gives DCPS an unfair edge. It allocates funding to DCPS schools based on overly optimistic enrollment estimates but doesn’t reclaim the money if enrollment winds up lower. Charter schools, meanwhile, receive payment only for verified enrollment.

Despite the School Reform Act’s clarity and a legislative history that confirms that Congress intended that funding to be uniform across district-run and charter schools and cover all operating expenses of DCPS, a federal judge ruled in September that non-formula funding for DCPS does not violate the law. She said the charter schools that brought the lawsuit have no standing to challenge how DCPS calculates enrollment.

Unchallenged, that decision would entrench unfairness that discriminates against 4 in 10 children who attend public schools in Washington, including some of the city’s most vulnerable. Half of D.C. charter school students are classified as “at risk” — homeless or in foster care, eligible for government financial and nutrition assistance, or overage and under-credited. Four in five charter students are economically disadvantaged, a higher share than in DCPS.

Before the School Reform Act allowed charter schools to operate in the District, half the city’s students never graduated. Two decades later, graduation and college-going rates are climbing, and test scores are up across grades and student backgrounds. Charters have led the way as results have improved in both sectors. Observers hold up D.C. as a model for how school choice can improve a city’s education system. Consider how much more progress we could make if we funded schools equitably.

Mayor Muriel E. Bowser’s (D) administration took a step toward equality when it announced that new funds for DCPS teacher salaries would be distributed through the per-student formula, ensuring that charter students were not disadvantaged by the city’s new contract with the Washington Teachers’ Union. But by continuing to contest the funding lawsuit and refusing to bring its practices into line with the law, the D.C. government shortchanges charter students who are no less deserving than their friends in DCPS schools.

The principle of fairness enshrined in the School Reform Act is designed to treat children in our city’s public schools equally, because just as at the park, everyone plays better when the field is level.