WASHINGTON — North Carolina and Tennessee will be allowed to change the way they measure student progress under the federal No Child Left Behind law, the only two states chosen for a national experiment.

The Education Department announced yesterday that the two states may track how individual students perform in math and reading over time, known as a "growth model."

Until now, states could only measure success or failure by comparing the scores of different groups of children from one year to the next. Many educators say that system is unfair because it does not recognize changes in the population or improvement by individual students.

Education Secretary Margaret Spellings also announced changes involving tutoring and schools affected by Hurricane Katrina, part of a campaign to show flexibility under the law.

How progress is measured is hugely important to schools, because it helps determine whether they meet their goals — and avoid penalties — under the education reforms passed by Congress in 2001. States have been clamoring for a different way to judge student success.

Of the 13 states that sought approval to track individual students over time, only two were approved. Spellings had planned to allow up to 10 states in the pilot project this year, but other states didn't qualify, mainly because they lacked sufficient data.

"We set a high bar, admittedly, because we really wanted to get good information," said Henry Johnson, the assistant secretary for elementary and secondary education. "I certainly would have liked to have had more."

Six states that had made the final cut were rejected: Alaska, Arkansas, Arizona, Delaware, Florida and Oregon. Five other states that applied — Colorado, Indiana, Iowa, South Carolina and Utah — were rejected earlier because their proposals did not meet the terms Spellings set out last fall.

North Carolina and Tennessee schools will be able to choose how they measure student progress — the old way or the new.

The two states were picked, officials said, because they have data systems to track individual students — including all children in required testing — and they committed to closing achievement gaps between whites and minorities. The two states will also track whether children who are already at grade level are still improving.

Every state must get every child up to par in reading and math by 2014.

Under the law, schools are required to test students in math and reading in grades three to eight and once in high school. Schools must show yearly progress overall and among groups of students as defined by race, disability, English language ability or economic situation.