Good teaching can make a dramatic impact on learning, but states and school districts have been slow to remove bad teachers from classrooms, reward good teachers and more equitably distribute effective teachers to low-income and predominately minority schools, education advocates and researchers say.

Little wonder, they say: Most states and districts don't even have reliable systems for distinguishing good teachers from bad.

"Incredibly, you can walk into almost any school in America, go down the hall to the first couple of classrooms you find, look at the teachers inside and realize this: Nobody -- not the principal, not the parents, not the students, not even the teachers themselves -- actually knows how effective these teachers are in helping their students learn," said a 2004 report by Kevin Carey, then director of research policy for The Education Trust.

"They probably have an opinion, maybe even some anecdotal evidence," Mr. Carey said. "But in terms of accurate, verifiable information about how effective individual teachers are at helping each of their students learn and make process from the beginning of the school year to the end? In the vast majority of schools, nobody knows."

This is the case despite evidence that effective teachers matter more than other factors influencing student achievement.

In a 1996 study, researchers at the University of Tennessee determined that students assigned to highly effective teachers for three years performed significantly better than peers assigned to other teachers for the same period.

Less than a decade later, researchers studying students in Texas determined that five years with highly effective teachers would go a long way toward eliminating the achievement gap for low-income seventh-graders.

The five-year-old federal No Child Left Behind Act, the sweeping education law driving school reform nationwide, holds states and districts accountable for student achievement. But it doesn't speak directly to the issue of identifying effective teachers or outline the steps schools should take with ineffective ones.

Not yet, anyway. Sandy Kress, who helped write NCLB as a senior education adviser to President Bush, said he believes debate over the law's reauthorization this year will include a new focus on teacher effectiveness.

The Education Trust, other groups and researchers have recommended analysis of individual students' test data so administrators can see how much kids grow over a period of time -- and how effective their teachers have been.

Advocates' other proposals are to link pay and performance, give highly effective teachers incentives to work in disadvantaged schools, involve parents in teacher evaluations and show more backbone with bad teachers.

"Even though school districts currently enjoy considerable discretion to discharge ineffective teachers during the first two years, they rarely do so," the Brookings Institution's Hamilton Project said in a report last spring. Researchers proposed that districts make it a policy to deny tenure to the 25 percent of new hires deemed least effective.

Sweeping change invites opposition from harried administrators and teacher unions.

So far, most states, districts and schools have been taking more measured steps to improve the hiring, training and development of faculty:

Quaker Valley School District's "Hire the Best" program, implemented in the mid-1990s, requires candidates to give demonstration lessons and sit for multiple job interviews.

The state requires a performance evaluation of new teachers twice a year and veteran teachers once a year. At Propel Schools, a charter school organization based on the South Side, teachers are evaluated three times a year and subjected to additional impromptu evaluations.

California last year enacted a law to halt the so-called "dance of the lemons," the shuffling of ineffective teachers from one school to another.

Instead of following termination procedures, some principals over the years would "gently suggest the person move on" to another school, said state Sen. Jack Scott, D-Pasadena, the law's sponsor. Now, a school the person wants to go to may turn down any teacher's transfer request, even if the teacher has seniority and is otherwise eligible to move.

"It might serve as a catalyst in other parts of the nation," said Mr. Scott, who took criticism from teachers unions for pushing through the measure.

The law also set an April 15 annual deadline for teachers to voluntarily transfer from one school to another. Mr. Scott said the transfer "merry-go-round" lasted long into the summer in some districts, depriving administrators of the opportunity to recruit and hire new teaching talent.

Guy Strickland, author of the book "Bad Teachers: The Essential Guide for Concerned Parents," said it takes too long and costs too much money for districts to fire many teachers. When teachers are let go, he said, it's more likely for poor judgment or bad classroom management than ineffective instruction.

The American Federation of Teachers says the problem rests with administrators who don't properly evaluate teachers -- and thus don't know whether they're doing a good job and may give good ratings to bad teachers.

Principals may use academic snapshots -- scores on state tests, for example -- to guide their assessments. But without analysis of individual students' growth over time, evaluations depend much on administrators' discretion, something that's true in higher education, too.

Under Pennsylvania law, new teachers must be evaluated six times during the first three years on the job and are subject to termination after one bad evaluation. Veteran teachers are subject to dismissal after two bad evaluations.

The Pittsburgh school district, which has about 2,600 teachers, moves to terminate about 12 a year. Most elect to resign rather than challenge dismissal, said Jody Buchheit Spolar, the district's director of employee relations and organizational development.

Kenn Marshall, spokesman for the Pennsylvania State System of Higher Education, said faculty members at 14 state-run universities must demonstrate competence during a six-year tenure process.

Even after receiving tenure, professors can be dismissed. But it's a more difficult proposition at that point, Mr. Marshall said.

A parent's gut feeling about a problem teacher may not be right. But it might be impossible to ignore, too.

Richelle Patterson, the AFT's assistant director for educational issues, said parents should talk first to the teacher to get a better handle on what's happening in the classroom. Next, she said, parents should discuss concerns with the principal and ask what supports the district may offer the teacher.

Mr. Strickland said there are other steps parents can take to increase the likelihood that their children will get good teachers. Parents of a third-grader, for example, should survey parents of older pupils to guide the selection of a fourth-grade teacher for the next school year.

He said principals, feeling the need to protect their employees and cultivate loyalty, get defensive when parents complain about a lemon. And he said a principal who admits to having one potentially must accommodate every parent who demands a child's reassignment to another classroom.

Better, Mr. Strickland said, for parents to tell the principal that their child and the teacher aren't a "good fit" and suggest that the child would be a "success story" in somebody else's classroom. That approach allows the principal to save face and appeals to his or her desire to promote achievement.