The article doesn't actually dispute that the difference is teacher quality. The article just claims that teacher quality can be improved without doing battle with teacher unions. There's a nationwide struggle to get high quality teachers into low income schools that I don't think anyone on either side of the ed reform debate would deny.
Additionally, as a teacher in a low-income school, I'd argue very strongly that you should replace "student quality" with "student attitudes." The latter doesn't suggest that poor kids can't learn.
I don't agree that the article thinks teacher quality is the difference.
Here is what the article says about altering teacher quality:
Most education researchers, though, recognize that Rhee's simple vision of heroic teachers saving American education is a fantasy...
If the ability to fire bad teachers and pay great teachers more were the key missing ingredient in education reform, why haven't charter schools, 88% of which are nonunionized and have that flexibility, lit the education world on fire?
New York didn't get ahead by firing bad teachers.
On student quality:
Rhee knew that attracting more middle-class students of all races into public schools would strengthen the schools for all students. In one interview, she recounted Warren Buffett's advice to her that the nation's education problems would be solved if private schools were made illegal and students were randomly assigned.
In D.C., the goal of making all schools majority middle class is not immediately possible, given that 63 percent of the city's students are low-income, but Rhee could have made significant progress in many schools for three reasons.
I won't replace "student quality" with "student attitudes" because I don't know if the relevant factor is attitude.
Additionally, as a teacher in a low-income school, I'd argue very strongly that you should replace "student quality" with "student attitudes." The latter doesn't suggest that poor kids can't learn.