Nearly half of New York City teachers reaching the end of their probations were denied tenure this year, the Education Department said on Friday, marking the culmination of years of efforts toward Mayor Michael R. Bloomberg’s goal to end “tenure as we know it.”

Only 55 percent of eligible teachers, having worked for at least three years, earned tenure in 2012, compared with 97 percent in 2007.

An additional 42 percent this year were kept on probation for another year, and 3 percent were denied tenure and fired. Of those whose probations were extended last year, fewer than half won tenure this year, a third were given yet another year to prove themselves, and 16 percent were denied tenure or resigned.

...In New York City and many other districts, tenure decisions are increasingly based on how the teachers’ students score on standardized tests, as well as mandatory classroom observations by principals or other administrators.

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