The middle class is the natural market for parochial school, particularly in cities like Philadelphia. Tuition is reasonable and the education is often better than public-school alternatives.
The church's problem - make that one of many, many problems - is that the middle class is shrinking across the country, and the decline is particularly acute in the Philadelphia region.
In November, researchers at Stanford University released a disturbing study documenting the erosion of mixed-income and middle-class neighborhoods, a phenomenon that has turned communities into increasingly segregated neighborhoods of rich and poor.
Nowhere, they found, was the trend more pronounced than in the Philadelphia metropolitan area. In 1970, when the region's middle class was starting to feel the strain, the area ranked 43d in income segregation. Now it ranks third.
That is the dynamic that Mayor Nutter warned against in his second inaugural address, when he said, "We must reject this notion of 'a tale of two cities.'
"A society divided into the rich and the poor, the affluent and the oppressed, the educated and the enslaved," Nutter said. "We are not two cities. We must not become two cities."
I share the sentiment. There is nothing healthy about a city split between elites and a permanent dependent class, a city whose children attend either the $27,000-a-year Friends Select or the local public Promise Academy.
But what can Nutter or the city realistically do to stop the growing inequality? The forces working against the middle class aren't isolated to Philadelphia: global competition, a financial system rigged in favor of the 1 percent, the labor movement in decline, and so on.
The latest census figures give us a glimpse of what Philadelphia's increasingly inequitable future looks like: the small trickle of people moving to the city tend to be privileged (well-off empty nesters, young professionals on the make) or poor (immigrants, the only reason Philadelphia reversed 50 years of population decline). Who's leaving? The white middle class, as always, but also a growing number of African Americans with means.
All of this is already obvious to anybody who lives or works in Philadelphia. Despite the mayor's hopeful insistence otherwise, the city is ever more divided between those who live here because they have no other option and those who can afford the expanding bubble of Center City.
This is increasingly the norm in big American cities. In wealthier cities, like San Francisco and Boston, the middle get priced out and a relatively small number of dependent poor remains, either in public housing or in tenaciously impoverished neighborhoods.
But Philadelphia is not a wealthy city. If the middle class dies out here, the downside will be far bleaker than an overabundance of bistros and art galleries.
Which is why the mass killing of Catholic schools is such a worrisome development, a not-so-early indicator of Philadelphia's inequitable future.
Patrick Kerkstra is a freelance journalist and former Inquirer staff writer. Follow him @pkerkstra on Twitter.