New York (CNN) -- New York City Education Chancellor Joel Klein will step down as the head of the nation's largest public school system, Mayor Michael Bloomberg announced Tuesday.
"He (Klein) has been a tireless champion for children and a fearless proponent for reform," Bloomberg said at a press conference. "Joel has implemented innovative changes that have made an enormous difference in the lives of millions of children."
Klein, who held the post since 2002, was the city's longest-serving education chancellor and oversaw 1,600 schools with 1.1 million students, 136,000 employees, and a $21 billion operating budget.
Klein said he will enter the private sector as an executive vice president at News Corporation.
"For more than eight years, I've been privileged to wake up each morning knowing that I had the opportunity to serve this city and a school system I love so much," he told reporters.
Under Klein, New York City saw a sharp increase in the number of charter schools -- a sometimes controversial system in which schools receive a mix of public and private money but operate independently.
His replacement, Cathie Black, is leaving her position as chairman of Hearst Magazines. She will be the first female serve as of education chancellor in New York City.
"All of us would agree that there is no higher priority in America than our educational system and the willingness to climb into the trenches in the fight for improvement is something to be greatly admired and supported, " Hearst Corporation Vice Chairman and CEO Frank A. Bennack, Jr. said in a statement.
Black is expected to assume the role of chancellor in the beginning of 2011 with Klein helping her with the transition, Bloomberg said