With New York City Mayor Michael Bloomberg on a mission to “end tenure as we know it,” nearly half of the city’s eligible teachers were denied the status that educators’ advocates embrace as essential for defense against discriminatory firing.

The rejections are the result of rigid tenure requirements put in place last year. New York City runs the nation’s largest public school system, employing roughly 75,000 teachers. Of the 5,231 teachers hired in the 2008-09 school year, 2,186 succeeded in gaining tenure, the United Federation of Teachers reports.

According to new rules, tenure decisions in New York City and many other districts are increasingly based on student performance on standardized tests. Bloomberg’s effort is a regional continuation of the ideas that underpinned President George W. Bush’s 2001 No Child Left Behind Act, namely, the belief that public education is best improved by punishing teachers for students’ failure to meet established performance standards, with emphases on showings in math and science. Performance is measured via annual statewide standardized tests.

The program compels teachers to teach to those standards under threat of losing their jobs and a loss of district funding. Pennsylvania has considered tying teachers’ salaries to test scores, and Ohio passed a law in July linking teacher pay to student performance.

Nothing in such programs accounts for economic and other social circumstances — such as poverty — that have been shown to profoundly affect students’ performance. Teachers shoulder all of the blame and burden.

In New York City, in addition to test performance, two other categories are used to determine teacher competence: “The teacher’s practice, based in part on classroom observations … and the contributions the teacher makes to the school community.” The city’s Education Department has a team that trains principals to make the assessments, which results in ratings of “highly effective, effective, developing or ineffective.” No hard rule exists for determining how many “effectives” or “highly effectives” are needed to gain tenure.

The result is an educational system that treats teachers like the children they serve, premised on the simplistic and ignorant notion that teachers alone — and not the complex, almost always limiting social circumstances in which they work — bear the responsibility for student success.

“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.”