When Terry Grier was hired to run the San Diego Unified School District in January 2008, he hoped to bring with him a revolutionary tool that had never been tried in a large California school system.

Its name -- "value-added" -- sounded innocuous enough. But this novel number-crunching approach threatened to upend many traditional notions of what worked and what didn't in the nation's classrooms.

Teacher evaluations: An article in some editions of the Oct. 18 Section A about evaluating teacher performance reported that the San Diego teachers unions spent nearly $400,000 in this fall's school board elections. The elections were held in 2008. —

Rather than using tests to take a snapshot of overall student achievement, it used scores to track each pupil's academic progress from year to year. What made it incendiary, however, was its potential to single out the best and the worst teachers in a nation that currently gives virtually all of them a passing grade.

In previous jobs in the South, Grier had used the method as a basis for removing underperforming principals, denying ineffective teachers tenure and rewarding the best educators with additional pay.

In California, where powerful teachers unions have been especially protective of tenure and resistant to merit pay, Grier had a more modest goal: to find out if students in the district's poorest schools had equal access to effective instructors.

Still, it proved radioactive to San Diego's teachers union. Like many unions across the country, it saw the approach as a flawed instrument, a Trojan horse for introducing merit pay and a threat to hard-won employment protections.

After nearly two years of grinding battles with the union and school board on this and other issues, Grier recently left for Houston, where the district uses value-added results as a basis for teacher bonuses.

The opposition in San Diego, Grier said mildly, was "more entrenched than I thought it would be."

His fight there offers a preview of a debate that is about to engulf the nation's schools.

The Obama administration has made value-added a pillar of its school-reform efforts, including the $4.35-billion federal grant program known as Race to the Top, which requires states to link student scores to teachers.

The administration's endorsement thrust to the fore a 20-year-old idea that has long been confined to academic circles and a handful of states and school districts, including Chicago's, where it was championed by now-Secretary of Education Arne Duncan.

Gov. Arnold Schwarzenegger this month signed into law two measures that put California on the path to measuring student growth with the value-added method.

But most districts thus far have just begun to talk about using the method in teacher evaluations.

Value-added analysis promises to address one of public education's central conundrums: On one hand, research shows that effective teachers are the single most important factor in improving student performance. On the other, most states use subjective evaluation systems -- based on occasional classroom visits by administrators -- that give nearly all teachers a satisfactory rating.

"Zero percent [of teacher evaluation] is based on student achievement," Duncan said in a recent interview. "That's a problem."

Protection of tenure

With no objective measure of success, firing a tenured teacher for incompetence is nearly impossible in many districts.