Students arrive for class at Washington Latin Public Charter School in Northwest Washington in 2015. (Allison Shelley/For The Washington Post)

A four-year-old lawsuit challenging the District’s treatment of charter schools continued in court Monday, with a coalition of charters arguing to federal judges that the city’s unequal funding of schools is breaking the law.

A federal judge ruled against the D.C. Association of Chartered Public Schools in 2017. The hearing Monday in the U.S. Court of Appeals for the District of Columbia Circuit represented the latest turn in a long-running legal battle over whether the city’s funding of education for more than 90,000 students is equitable.

Nearly half the District’s public school students are enrolled at charters — which are publicly funded but privately operated.

If the charter schools win their appeal, the city could have to rethink how it allocates more than $1.5 billion in education spending.

“Since the inception of public charters, charters have only been asking that the D.C. government not privilege one sector over the other sector,” said Irene Holtzman, the executive director of FOCUS, a charter advocacy organization. “This is an opportunity to do right by all students in the District.”

D.C. uses a uniform funding formula to allocate money to traditional public and charter school students. The money varies depending on students’ needs but not on where they attend school.

The city allocates $3,124 more per student to charter schools than to traditional schools, with that money earmarked for facility expenses. Charters are responsible for acquiring and maintaining their school buildings, while the city provides buildings to D.C. Public Schools.

But the lawsuit argues that the traditional public school system relies on other government agencies to provide some services, meaning those costs don’t come out of the school system’s budget. For example, the Department of General Services provides facility maintenance, and the Office of the Attorney General for the District of Columbia provides legal services.

City officials and education activists have argued that fundamental differences exist between the two education sectors and that the traditional public school system requires subsidies because it must maintain enough space and staff across the city to serve any student who shows up throughout the year. Charter schools do not have that same obligation.

Unlike charters, D.C. Public Schools is also required to use union labor, which is often more expensive.

Cathy Reilly, executive director of the Senior High Alliance of Parents, Principals and Educators, a D.C. advocacy organization, said the charter schools have not persuasively argued that their students are harmed because of the way the money is distributed.

D.C. Public Schools “has different responsibilities as a city agency responsible for welcoming students into their school of right, no matter when and no matter how many,” Reilly said.

But the appeal hearing Monday in front of three federal judges — two Barack Obama appointees and one George H.W. Bush appointee — primarily focused on whether the charter schools had the right to sue the city in federal court.

Congress passed the D.C. School Reform Act in 1995, opening the door for charter schools in the District. The law required the city to set up a uniform formula to fund charter and traditional schools equally on the basis of enrollment.

The attorney for the charter schools, Kelly Dunbar of the law firm WilmerHale, argued that the city is bound by federal law and must provide equal funding for all public schools in the city. Federal law, he argued, should supersede local law.

The original lawsuit filed by the charter schools argued that between 2008 and 2014, the city spent about $2,150 less per charter student each year than it did on traditional public school students after taking into account the services other city agencies provided to the traditional system.

The lawsuit has been viewed as a test of the city’s authority to govern itself using powers delegated to the District by the Home Rule Act of 1973.

The city’s attorney, Jason Lederstein, argued that it is a local matter and that the Home Rule Act allows the District to make its own decisions about how to fund its schools.

“You can’t come to a federal court and say there was a violation of the School Reform Act,” Lederstein said in court Monday.

Dunbar said that if the judges were to rule against the charter schools on jurisdictional grounds, the schools could file a lawsuit in local court.

“Were we to lose on the jurisdictional point, that should have the effect of wiping the slate clean, and these questions could be subject to litigation in D.C. Superior Court,” Dunbar said in an interview.