Urban flight isn't just for whites anymore.

Middle- and upper-middle-class blacks are emptying out of big Northern cities - Philadelphia, Chicago, New York, Cleveland - in huge numbers. Some are heading south for warmer weather and more abundant jobs. But a lot are relocating just a few miles away, in the suburbs, and for all the old reasons. Safer neighborhoods. Better schools. Parking. Central air.

In all, the black population in Philadelphia's Pennsylvania suburbs grew 32 percent over the last decade, as reported by the website Metropolis. That's a major shift. And the Trayvon Martin tragedy is hardly the first indicator that the integration of the suburbs hasn't been friction-free.

Last month the federal Department of Education released data from 72,000 schools across the country that showed black students are disciplined more frequently and more harshly than other students. Intrigued, I looked at the local data.

Few will be surprised to hear that in Philadelphia black students are disciplined - with punishments such as suspensions and expulsions - more frequently than white and Asian students.

But look to suburbia for the most glaring cases of racially unequal student punishment in the Delaware Valley.

Black public school students in Bucks, Chester, Delaware, and Montgomery Counties make up just 14 percent of the total student body in the 53 local suburban school districts surveyed by the Department of Education. And yet they accounted for more than half of all serious suspensions in the suburbs.

Here's another number that puts these massive disparities in perspective: A black public school student in Philadelphia's Pennsylvania suburbs is 12 times as likely as a white student to be suspended more than once. That blows away the disparity in Philadelphia, where black students are 2.8 times as likely as white students to get suspended more than once.

The gap between the discipline of white and black students in the city's suburbs is huge compared with the national average as well: Across the United States, black students account for 18 percent of the student body, and 46 percent of those students suspended more than once.

When he released the data, U.S. Education Secretary Arne Duncan said: "The undeniable truth is that the everyday educational experience for many students of color violates the principle of equity at the heart of the American promise."

So what does it say that in Philadelphia's suburbs the disparities top even the depressing national numbers? Are we to believe that black students in Philadelphia's suburbs are really that much more poorly behaved than either their white counterparts or black students in the rest of the country? Or are these figures evidence of some seriously troubling suburban teacher and administrator bias against black students?

The American Civil Liberties Union is in the middle of a comprehensive analysis of discipline data statewide that goes way beyond my number-crunching. ACLU research leader Harold Jordan hopes that the survey - which included filing right-to-know requests with every last district in the commonwealth - will help explain why these disparities exist, and not just document how bad they are.

For now, though, Jordan has a few well-grounded hunches. A big one is that, for a lot of uncomfortable reasons, black students still stand out in the suburbs.

"In some suburbs, I believe there is a hyper-visibility issue for a lot of African American students," Jordan said. "They don't look like everyone else."

That can mean that some teachers might overlook a misbehaving white sixth grader while across the room a black student of the same age doing the same thing might be seen as a "proto-criminal," Jordan said.

Yet if standing out were the only factor, the discipline disparities would be comparably large for Asian and Hispanic students. But they're not. The new data show that Hispanic students are indeed punished more than white students in the Philadelphia suburbs, but at nothing like the rate of black students. And Asian students receive fewer suspensions and expulsions on average than white students.

Jordan's other theory is that some leaders of suburban schools are scared that their sanctuaries will be infected by big-city ills. So some are extra vigilant, coming down hard on those students who might be suspected of bringing too much city to the suburbs: black students, in other words.

"In the 'burbs, there's a notion that the schools have to be kept purer. There's a sense that 'we don't tolerate city discipline problems here,' " Jordan said.

Maybe not. But the huge racial disparities the federal numbers reveal strongly suggest that suburban schools do have a problem. I'm just not sure it's the students that are to blame for this one.

Patrick Kerkstra is a freelance journalist and former Inquirer City Hall reporter. He can be reached Patrick@PatrickKerkstra.com or on Twitter @pkerkstra.