Slightly more than half of Roanoke students graduated on time with a standard or advanced high school diploma this spring, according to new information released Wednesday by the Virginia Department of Education.

This is the first year that state officials have released data under the more accurate cohort method of calculating graduation rates. It shows that 51.6 percent of the city's high school seniors graduated in four years with either of those diplomas, suggesting that Roanoke schools are still struggling to boost their graduation rates. When counting students who earned a modified standard diploma, a special diploma or a general achievement diploma -- designed for students with disabilities or who are overage -- the graduation rate comes to 58.7 percent.

Roanoke is one of only four districts in Virginia with a graduation rate below 60 percent.

The city's rate for black students, 57.4 percent, lagged behind the rate for white students by three percentage points, an indication that the well-documented achievement gap between white and black students persists all the way to graduation.

The cohort method assigns a number to every student, allowing officials to track each individual student's high school career and come up with a reliable graduation rate. Until now, it had been only an estimate.

The numbers also underscore Roanoke Superintendent Rita Bishop's efforts last year to open Forest Park Academy, designed to help struggling students achieve a standard or advanced studies diploma.

Statewide, the rate of students who graduated with one of five diploma types recognized by the state stood at 81.3 percent.

The achievement gap was pronounced at the state level, with the graduation rate for white students surpassing the graduation rate for black students by almost 13 percentage points.

Roughly 77.4 percent of Virginia's students graduated with a standard or advanced diploma.

Roanoke's Patrick Henry High School graduated 47.7 percent of seniors on time with a standard or advanced high school diploma last spring. When considering those students who earned one of the two diploma types for students with disabilities, the graduation rate stood at 58.5 percent. At William Fleming High School, the graduation rate stood at 56.3 percent and 59.8 percent, respectively.

In 2006, when the graduation rate was calculated under the less reliable method, state education officials estimated that 56 percent of Roanoke students graduated on time with a standard or advanced diploma. That rate had hardly budged in 2007 to 57 percent.

"What I worry about are the kids without the high school diploma," Bishop said. "We're now doing a much better job of tracking them, finding them and getting them reattached."

Bishop said she expected the graduation rate to improve next year. "I'm very hopeful that we've made some very positive moves for the community."

Only the standard and advanced diplomas are counted when determining whether school districts meet federal standards under the No Child Left Behind Act of 2002, although federal education officials still use the older, less reliable formula. State officials have lobbied the U.S. Department of Education to recognize other types of diplomas for students with disabilities.

State Superintendent Patricia Wright said state education officials would continue to offer help to school districts concerned about their graduation rate.

"You can't change a graduation rate overnight," she said.

In Roanoke County, almost 90 percent of students earned one of the state's five diplomas. The graduation rate in Salem was 85.8 percent, according to the state. In Montgomery County it stood at 79.6 percent, in Franklin County at 77.3 percent, in Bedford County at 87.1 percent, in Botetourt County at 87 percent and in Pulaski County at 76.8 percent.

As of this summer, 16 states had already reported graduation rates using this new formula, according to the National Governors Association, which committed three years ago to use the new formula nationally. Virginia is one of five states that had been scheduled to release results this fall. Almost every other state is in the process of adopting the new formula.

The U.S. Department of Education is also considering requiring that all states use the formula for federal accountability purposes, which would allow researchers and policymakers to study graduation rates across states.

"This formula gives us a much more accurate picture of graduation rates in Virginia," Wright said.

Daria Hall, a policy researcher for Education Trust, a Washington-based education policy group, said the new method is "a huge step forward."

But the graduation rate is only as accurate as the numbers used to calculate it, she said. "There needs to be a really active role in state leadership and district leadership in ensuring that the quality and accuracy of the data that gets fed into this calculation."

Wright said state officials have been working with school administrators "for over a year" to make sure the data were being properly collected.