A QUESTION NOT completely answered with the release last month of state scores on rigorous national assessments centered on how much of the District’s improvement could be traced to the public school system. Could it be, as some skeptics suggested, that the District’s thriving public charter schools were responsible for the remarkable growth?

Now we have an answer. Any doubt about the progress being made by the public school system — and the efficacy of its hard-won reforms — was erased last week by new data showing D.C. Public School (DCPS) to be the system with the greatest improvement of any urban district in the nation. Analysis of the National Assessment of Educational Progress (NAEP) of 2013 urban district test scores showed the District to be the only city with statistically significant growth in both reading and math, and at both grade levels tested.

The gains of 5 points to 8 points since 2011 are the highest scores ever shown by DCPS students on these tests. Additional analysis, The Post’s Emma Brown reported, showed the system’s gains equaled or exceeded those of the city’s public charter schools in each tested subject and grade level.

And the results are not merely a reflection of the city’s changing population; analysis by the Council of the Great City Schools shows that demographic changes alone do not explain the growth in student achievement. While lagging behind the big-city national average, the District has moved up in ranking and is no longer at the bottom.

As Chancellor Kaya Henderson noted, D.C. schools are still far from where they need to be. Too many students can’t perform at grade level, and there is a significant achievement gap between white and minority students. But the progress seen in the NAEP scores, which are consistent with other measures, is an encouraging sign that the challenges can be met by undaunted leadership. “We’re coming on strong,” Ms. Henderson said, “and if we’re ever going to get where we need to go, it’s this kind of leapfrogging growth that is going to get us there.”

Credit goes to the District’s students, teachers and principals, whose hard work is reflected in the scores. Their success in turn depends on steady leadership. It is important that the District not stray from the course of school reform started in 2007 with mayoral control. Change is difficult, particularly in fraught areas such as closing underused schools or firing incompetent teachers. But tougher standards, strict accountability, consistent leadership and improved instruction are yielding measurable progress for D.C. students. That is what should count most.