ATLANTA - An extensive analysis of Georgia standardized tests is raising fears that the exams may have been altered by teachers or administrators worried about facing sanctions under the federal No Child Left Behind law.

The analysis found that 1 in 5 elementary and middle schools in Georgia submitted highly abnormal answer sheets last year.

The state education board last week ordered districts to investigate the cause of the irregularities in 191 suspect schools. In one extreme case, an Atlanta middle school was flagged for abnormally high incidents of changed answers in 89.5 percent of its classes.

Officials stressed that no one has been accused of cheating. The investigation was conducted by CTB/McGraw-Hill, the state's testing vendor, and released last Wednesday.

A similar though less extensive investigation last year found that a small number of educators in four districts changed test scores, apparently to meet mandated yearly progress goals.

Gov. Sonny Perdue, in a statement to the state board last Thursday, said he was "very concerned" about the results of the current investigation and said it was unlikely they were caused by a freak of math: "The analysis was very generous in allowing for statistical anomalies," he said.

The analysis looked at scan sheets for students in first through eighth grades. It flagged classrooms in which the number of answers marked wrong - then erased and marked correctly - was higher than three standard deviations above the state average.

If educators are indeed at the heart of the odd scores, it would constitute one of the largest scandals of its kind since the passage of the 2001 No Child Left Behind Act, which mandates the replacement of administrators and staff members in underperforming schools.

Robert Schaeffer of the National Center for Fair and Open Testing - a group critical of the reliance on standardized tests - said the Georgia affair appeared to be part of a growing trend, based on anecdotes from around the country.

"There's no question that reports of cheating have increased," Schaeffer said, although he noted that the known extent of the problem is based on how vigorously the cheating is ferreted out.

Meanwhile, the Obama administration is preparing to roll out incentives for similar merit-pay programs nationwide.

Critics such as Schaeffer say that previous scandals have shown that the emphasis on test results tempts educators to alter scores to avoid sanctions. He said that tying teacher pay raises to scores - a proposal now being considered by Georgia lawmakers - will create another temptation.

"When test scores are all that matters, school personnel will feel forced to get them by hook or by crook," he said.

In a phone interview Friday, U.S. Education Secretary Arne Duncan said it was likely that "a tiny, tiny number of teachers" changed scores. If caught, he said, "you've got to get them out."

Duncan added: "I think Georgia is doing the right thing to address (this) head on, and deal with it."