Joel Klein wasn’t exactly the natural choice for chancellor of New York City’s gargantuan and stupendously dysfunctional traditional public school system when he got the job in 2002. After all, the former U.S. Assistant Attorney General was better-known for his successful antitrust case against Microsoft in the 1990s than for any forays into education. As it turned out, the school district — which was taken over by New York City Mayor Michael Bloomberg as one of the first major efforts to place school systems under mayoral control — needed someone who wasn’t fully ensconced in the  stale status quo of the traditional education establishment to set it on a course for the better.

Eight years later, as Klein steps down as head of the nation’s largest traditional public school system, one can say he has changed the district’s once-infamous culture of mediocrity, byzantine bureaucracy and shockingly banal corruption. Graduation rates for the district (based on eighth-grade enrollment) have increased from 56 percent for the Class of 2002 to 63 percent for the Class of 2008, according to Dropout Nation‘s analysis of federal and New York City data. Student achievement levels have progressed steadily even as New York State has revised its own state tests for greater rigor.  The district is still a work in progress; the city’s abysmally low graduation rates for black males (and the New York Post‘s report about the travails of Wayne Knowland (who graduated from Fannie Lou Hamer High School despite his functional illiteracy) shows that there is still more work to be done. But the city is still doing better for its kids than it did nearly a decade ago.

Meanwhile New York City has become the pioneer for concepts that are (sadly) still considered innovative for education. Its development of the ARIS system has given teachers and administrators a tool that can be used for identifying potential dropouts, improving instruction and fostering long-term connections between teachers and their students. The shutdown of the city’s worst dropout factories and their replacement with smaller high schools with new cultures and more-dynamic staff has also shown an alternative to the rather faulty model of school turnaround efforts being advocated by the Obama administration. And Klein’s relentless pursuit of teacher quality reform — including efforts to use student test data in evaluating newly-hired teachers (and keeping laggards from gaining near-lifetime employment through tenure), and the attempt last month to publicly release Value-Added performance data has helped galvanize school reformers and others around dealing with one of the most-crucial steps in stemming the nation’s dropout crisis.

But Klein’s tenure isn’t just notable for its results. It offers some lessons to school reformers everywhere: You can successfully overhaul a traditional school system — and take numerous tough steps — and still be amiable to allies and critics. Traditional districts can be top-notch authorizers of high-quality charter schools — and even play significant roles in fostering their development (and giving families escape hatches from the worst traditional public education offers). And he has proven that education should not be left to teachers, principals and traditional players alone. We will not revolutionize American public education until we create dynamic cultures that embrace the genius within all of our children.

Cathleen Black, who succeeds Klein as chancellor, will have to do as good a job as he has. She has no choice but to succeed. Our kids need her to follow upon Klein’s stellar work revamping the Big Apple’s school system.

Watch this Dropout Nation excerpt of Klein’s speech earlier this year about turning around New York City’s high schools