(Photo: Patrick Giblin / Flickr)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.

The totals reflect a reversal in the way tenure is granted not only in New York City but around the country. While tenure was once considered nearly automatic, it has now become something teachers have to earn.

A combination of factors — the education reform movement, slow economies that have pinched spending for new teachers, and federal grant competitions like Race to the Top that encourage states to change their policies — have led lawmakers to tighten the requirements not only for earning tenure, but for keeping it.

Idaho last year did away with tenure entirely by passing a law giving newly hired teachers no expectation of a contract renewal from one year to the next. In Florida, all newly hired teachers now must earn an annual contract, with renewals based upon their performance.

Last month in New Jersey, Gov. Chris Christie signed legislation overhauling the nation’s oldest tenure law and making it easier for teachers to be fired for poor performance.

“There has been a sea change in what’s been happening with the teacher tenure laws,” said Kathy Christie, a senior official with the Education Commission of the States, a policy organization funded by state fees and grants. “In 2011 there were 18 state legislatures that addressed some component of teacher tenure and many of them in a significant way, and that is enormous.”

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.

“It is an important movement because what we know is that when schools improve, a lot of the improvement relates back to having really strong teachers organized around a common vision,” said Shael Polakow-Suransky, the city Education Department’s chief academic officer. “I think New York City has some of the best teachers in the country. It is a good place. People want to be here. So we are very fortunate. But we also want to keep pushing them, just like we want to keep pushing our kids.”

Tenure does not afford any advantages in pay or job assignments, or guarantee permanent employment. Its most important benefit is to grant teachers certain protections against dismissal without justification, including the right to a hearing before an arbitrator. Teachers and their unions embrace tenure as an important defense against indiscriminate or politically tinged hiring and firing.

Michael Mulgrew, president of the United Federation of Teachers, the city teacher’s union, said that he had always supported a “rigorous but fair” process of granting tenure. But, he said, large numbers of teachers were quitting the profession early in their careers, a sign that the city had not yet figured out how to help them succeed.

According to the union, of the 5,231 teachers hired in the 2008-9 school year, nearly 30 percent had quit by the end of their third years. There are roughly 75,000 teachers in New York City schools, the nation’s largest public school system.

“If New York City hopes to have a great school system, it will need to come up with better methods of helping teachers develop, not only at the beginning but throughout their careers,” Mr. Mulgrew said.

Mr. Polakow-Suransky said it was not uncommon in the United States for teachers to leave the profession in the first few years, when things are the toughest. Every new teacher in New York receives mentoring in the first year, as a “support system,” he said. “But if someone is not making it, and not happy, or the principal says, ‘You are not cut out for this,’ it is likely that they move on to something else, and that is not a bad thing,” he said.

Joel I. Klein, the former schools chancellor, began nudging principals several years ago to judge teachers more critically when deciding on tenure, and the percentage of denials slowly rose. But in 2010, when the mayor set about “ending tenure as we know it so that tenure is awarded for performance, not taken for granted,” 89 percent of teachers were still receiving it after their three-year probations ended.

The city’s Education Department now has a team that trains principals in gathering the kind of evidence needed to assess a teacher’s skills. It also developed a rubric in which teachers were rated on a four-point scale in each of three categories: the teacher’s practice, based in part on classroom observations; students’ learning, which is judged largely on test score improvement; and the contributions the teacher makes to the school community.

In each of those, teachers receive a rating of highly effective, effective, developing or ineffective, officials said. There is no hard rule on how many “effectives” or “highly effectives” are needed to gain tenure, which 2,186 teachers earned this year.

The new system began to take full effect last year, when only 58 percent of teachers gained tenure after three years, and an additional 39 percent had their probations extended. There is no limit to the number of years the city can extend a teacher’s probation, though officials of the Education Department and the union said they had not heard of any teacher receiving more than three extensions.

One special education teacher in Queens who was given a second one-year extension this year said that school officials cited improvements she needed to make but were short on details of what criticisms her principal had. “No specifics were ever given,” said the teacher, who spoke on the condition of anonymity for fear of retaliation.

Also, she said, the new tenure evaluations were dividing teachers and lowering morale, with some newer teachers feeling punished for the smattering of more experienced ones they saw as using tenure as a “safety net,” but putting forth less effort in the profession.“The bigger picture is that they are trying to end tenure,” the teacher said.

The nationwide shift on tenure has been remarkable for its speed and breadth, said Sandi Jacobs, vice president of the National Council on Teacher Quality. It was awarded “virtually automatically” in most states as recently as 2009, she said.

“Tenure was looked at as much more of a sacred cow,” Ms. Jacobs said. “Once states started to move on it, then the dominoes started to fall in other states.”

This article, “Many New York City Teachers Denied Tenure in Policy Shift,” originally appears at the New York Times News Service.