It would be hard to find a better example than this article of how Ravitch selectively uses data to present a distorted picture of the progress in NYC under Bloomberg and Klein (see Charles Sahm's rebuttal below):

For the past five years, Mayor Michael Bloomberg and Chancellor Joel Klein have claimed that, due to their programs, New York City was a national model. They proclaimed that the city had made "historic gains" on state tests, all because of the mayor's complete control of the policymaking apparatus. The mayor testified in congressional hearings that New York City had cut the achievement gap in half. Klein traveled to Australia to boast of the city's gains, and the Australian minister of education intends to align that nation's education system with the New York City model.

It was an exciting and wonderful ride while it lasted. But last week, with the release of the state test results for 2010, New York City's claims came crashing to the ground. The national model went up in smoke. The miracle was no more. The belief that mayoral control was a panacea for urban ills was no longer sustainable.

Secretary of Education Arne Duncan has gone around the nation for the past 18 months singing the praises of mayoral control. But in light of the New York City fiasco, he will have to find a new example when he lectures urban audiences, because the New York model just lost its wheels.

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Ravitch: Mayoral control means zero accountability

My guest is Diane Ravitch, New York University education historian and author of the best-selling "The Death and Life of the Great American School System." Ravitch, once a supporter of No Child Left Behind and now a fierce critic of its impact, is traveling the country and meeting thousands of teachers as she blasts the Obama administration's education policies.

By Diane Ravitch
http://voices.washingtonpost.com/answer-sheet/education-secretary-duncan/ravitch-mayoral-control-means.html