A new study of 270,000 public school students warns that the No Child Left Behind law may prompt some parents to send children from low-performing schools to others that appear to foster high achievement but do a poor job of raising individual student scores.

The study endorses the view of many educators, including some supporters of No Child Left Behind, that school achievement ratings will work better once all school systems can keep track of every student's improvement each year, rather than just compare one year's average test scores with the scores of other students from the previous year.

"While it is clear that our educational system should leave no child behind, it is also clear that the mission of our educational system needs to go beyond this goal," said the study by the nonprofit Northwest Evaluation Association based in Lake Oswego, Ore.

Officials at the U.S. Education Department said they will have to study the full report, available at www.nwea.org/research/growthstudy.html, before they could judge its worth.

The association used data from about 1 million U.S. students who took standardized tests at the beginning and the end of each school year to determine how much each student improved. Using that information, the association produced a growth index that measures how much each student learned in a year, compared with the average student.

The analysis shows that many schools with average test scores high enough to meet the annual milestones set by the No Child Left Behind law were below average in individual student academic growth, said Allan Olson, the association's executive director and one of the study's authors. More than 20 percent of schools with scores high enough to meet the federal targets "fall into the bottom quarter of schools in terms of the amount of growth they cause in their students," the study said.

The federal targets are measured as average passing rates on a test, and compare one group of students with a different group of students from the previous year. The study authors said this disguises and obscures how much each individual student learns during each year.

Under No Child Left Behind, schools that accept federal anti-poverty education funds must demonstrate "adequate yearly progress" toward ensuring that all students will be proficient in reading and math by 2014. Schools that consistently fail to do so must provide transportation for students who wish to transfer to higher performing schools.

If parents choose schools simply because they meet federal targets, they may find the transfer does not help their children, the study concluded.

However, Ross Wiener, policy director at the nonprofit Education Trust, said such a transfer might still be a good idea because a high-performing school is likely to set more ambitious goals for students than a lower-performing school would.