For the fourth year in a row, Roanoke schools have failed to keep hundreds of students until graduation. Newly released data from the Virginia Department of Education show that the graduation rate in Roanoke stood at 57 percent last year, up one percentage point from the 2005-06 school year but still considerably lower than surrounding districts.

The latest statistics came as a letdown to city school officials who have been working to stem the number of students who drop out.

"I always want and expect improvement," said school board Chairman David Carson. "Of course I'm disappointed."

But as dismal as the numbers are, Roanoke's problem may be even worse than advertised.

Virginia relies on the so-called "leaver" rate, one of several methods states use to calculate the graduation rate, as required under the federal No Child Left Behind Act. The leaver rate is based on the number of high school dropouts, a notoriously unreliable statistic because school districts rarely keep accurate numbers of dropouts.

To get a more accurate picture of the state's graduation rate, Virginia education officials are switching this year to a new method, the "cohort" rate to calculate how many students earn diplomas.

The cohort rate tracks individual students as they proceed through high school. It's the "gold standard" of graduation rate formulas, but it will also show a lower graduation rate, said Daria Hall, assistant director for K-12 policy with the Washington, D.C.-based Education Trust.

The cohort rate "is a much more accurate rate and as a result is likely to be lower," she said.

The National Governors Association adopted the cohort rate in 2005 to make it possible to compare graduation rates across states. Several states, such as Arizona and Massachusetts, have already put it in place and several more, such as Virginia, plan to do so soon.

"It's going to capture those students who transfer in and transfer out. It's going to be an actual graduation rate versus an estimate," said Charles Pyle, a spokesman for the Virginia Department of Education.

Also, at the federal level, U.S. Education Secretary Margaret Spellings said Tuesday she would end the states' patchwork of graduation rate formulas and require them all to use a single method, a sweeping change that, in some states, could produce vastly different graduation rates.

"We're going to see some cold, hard numbers that are going to be difficult to look at, but they reflect the reality of far too many young people in this country," Hall said.

Keeping students from dropping out of school has been a persistent problem in American education, one that has generated study after study. The latest came Tuesday, with a report showing that most of the country's 50 largest school districts struggled to graduate even two-thirds of their students.

The study, by America's Promise Alliance, a group founded by former Secretary of State Colin Powell, also found a large discrepancy between the graduation rate in urban school districts and in their suburban counterparts.

That discrepancy is particularly pronounced in the Roanoke Valley, where suburban Roanoke County posted a 90 percent graduation rate last year, 33 percentage points higher than in the city of Roanoke. Roanoke is not among the 50 largest school districts and was not part of the study.

"This is a crisis for that community, for those students, for their families and for the state as a whole," Hall said. "We've had this problem for a long time, and we're just now trying to confront this problem."

In Roanoke, the school system is working to open an academy for overage middle school and high school students. Superintendent Rita Bishop, in her first year on the job, is also working on recruiting and retaining teachers who can help at-risk students stay in school.

Carson, the school board chairman, acknowledged that the school system deserves some of the blame for the low graduation rate. But it will take more than the efforts of the school system to reduce the number of dropouts in the city, he said.

"No one is holding a gun to a dropout's head to drop out of school," he said. "Somewhere along the line it is deemed acceptable to do so, and we must, as a community, as caregivers, as parents, make that unacceptable.

"We absolutely need a cultural shift."