Pennsylvania has the nation's starkest spending gap between rich and poor school districts, U.S. Education Secretary Arne Duncan said Friday, and that must be remedied.

Statewide, poor districts like Philadelphia spend 33 percent less per student than wealthy districts, the biggest such gap in the country, according to data from the National Center for Education Statistics.

In the poorest quarter of Pennsylvania districts, current expenditures per student - with the exception of most federal funds - are $9,387. In the wealthiest fourth, districts spend $12,529, not counting most federal funds.

"The children who need the most seem to be getting less and less, and the children who need the least are getting more and more," Duncan told reporters Friday on a conference call.

Education spending in poor districts lags that of wealthy districts in 23 states, the secretary said. Altogether, 6.6 million children nationally "are being shortchanged" when it comes to state and local education funding, according to a department news release.

The spending gap has widened in the last decade, Duncan said.

The education secretary wants Congress to close a federal loophole that permits wealthier districts to receive more funding than needier districts, he said. The issue is front and center as lawmakers debate the reauthorization of the Elementary and Secondary Education Act.

"We still have school systems that are fundamentally separate and unequal," Duncan said.

Education funding is now a front-burner issue in Philadelphia and across the state. Gov. Wolf, in his first budget, has proposed millions in new aid to schools, and a committee is studying what a fair funding formula might look like. Pennsylvania is just one of three states in the country that lack such a formula.

Separately, a group of school districts, nonprofits, and parents, including the William Penn School District in Delaware County and two Philadelphia parents, have sued the state, alleging its education-funding structure is unconstitutional. Lawyers for both sides appeared in Commonwealth Court this week.

To highlight his message, Duncan is scheduled to visit E.M. Stanton Elementary, a strong Philadelphia public school in South Philadelphia, on Friday. He will be joined by Superintendent William R. Hite Jr., Pennsylvania Education Secretary Pedro Rivera, and Mayor Nutter, according to the district.