It's less a question of "suburban vs urban" than "wealth vs poverty."
Big cities have wealthy neighborhoods and poor neighborhoods, good schools and bad schools. And likewise, there are wealthy suburbs and poor suburbs.
A school that serves a mix of wealthy and poor suburbs probably has a more-average quality than the low end of a highly segregated city, but you can bet that a school in a tony part of NYC is going to be a hell of a lot better still.
All of this is due to San Antonio vs Rodriguez, which IMHO is the worst Supreme Court decision since Dred Scott.
I was wondering the same thing. How can they isolate walkability rather than cultural factors or factors related to the public school system of a city?
Public schools seem to be regaining credibility as the upper-middle class flood back into cities, but have they caught up? Unsure.