“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.