By Arne Duncan

As I watch what is happening in Philadelphia's public education system, I can only conclude that until some glaring funding injustices are fixed, in Philadelphia and in many school systems around the country, we will never live up to our nation's aspirational promises of justice.

Philadelphia represents one of the most vivid examples nationally of what happens when systems fail to fund schools properly. At Lingelbach Elementary School, Principal Marc Gosselin has an annual discretionary budget of $160 - and poison ivy climbing the edges of his classroom windows. Teacher Jason Chuong has to work part-time at seven different city schools, none of which can afford a full-time music teacher. And while many in Philadelphia's suburbs send their children to well-funded schools, the district is in such straits that several nonprofit organizations wrote Gov. Corbett in 2013, saying Pennsylvania has failed in its basic duties under the state's constitution.

"A system that fails to provide for the academic success of its students is not just an academic problem," the letter reads. "Today's students are tomorrow's workers, citizens, and taxpayers. To the extent we fail them today, Pennsylvania's communities will fail in the future."

These funding inequities fly in the face of the district's recent efforts to set its students up for success, with hard-won fiscal discipline, safer schools, higher standards, and new, innovative high schools. And these inequalities exist in far too many places in this country. Few question that education is the key to American growth and prosperity - yet too many systems value some children's futures more than others'.

In Kansas, the Supreme Court ruled in March that funding disparities among schools violated the state constitution. At the University of North Carolina, a member of the Board of Governors in July described her state's under-investment in education as a "grievous mistake" and an "embarrassment." These are not isolated examples.

In too many places nationwide, parents' income and real estate prices predict the quality of public education - and minority and low-income students attend schools that receive lower per-pupil spending. And significant spending inequities exist not just across district lines, but inside them as well.

It should embarrass all of us that such injustices endure. But the good news is that we can do something about it - if we get serious about changing the way we think about school funding.

Philadelphia's challenges are complex, as are the causes. In Pennsylvania, reform efforts began in 2007, when the state legislature released a study determining that its schools were being underfunded by more than $4 billion. Like most states, Pennsylvania raises the bulk of its money for schools via local property taxes. That means poorer communities have less money to work with - as a matter of law.

In 2008, Gov. Ed Rendell pledged to aggressively close the funding gap, and state officials introduced a new school funding formula that started awarding money based on a stronger focus on student needs. But over the last four years, we have instead seen a suspension of the funding formula and major cuts to the state's overall budget.

The impact has been devastating for districts like Philadelphia, where income levels are so low that four in five public school students qualify for a free or reduced-price lunch. As Philadelphia Schools Superintendent William R. Hite Jr. put it, "The fiscal landscape we have grappled with over the past several years does not, and cannot, reflect the value we place on young people and our collective obligation to them."

The Equity and Excellence Commission, established in part through the work of U.S. Rep. Chaka Fattah (D., Pa.), has reinforced eloquently the point that equity of funding is a national challenge and an urgent one.

I am hopeful that help is on the way. In June, Pennsylvania created a bipartisan commission of state legislators to develop a new funding formula that will account for each community's student population and needs. Gov.-elect Tom Wolf has vowed to undo funding cuts and to implement a fair funding formula that can assure greater equity. The key to a fair funding formula is quite simple: Target aid to students who need it most, and adjust current levels of state aid to the districts that are already well supported.

Moreover, I am pleased that the School District of Philadelphia has tightened its own fiscal management, improving a situation that nearly a decade ago hurt confidence in the city and led to an investigation by this department.

In Pennsylvania and nationally, getting funding right is a matter of fairness and justice, of ensuring that schools have adequate resources to do their vitally important work. But beyond that, equitable funding reinforces our founding values as a country and signals to all citizens how seriously we take our commitments to one another.

That's why we must do better. Perhaps Philadelphia can show us the way.

Arne Duncan is the U.S. secretary of education.