George Johnson is not looking forward to this week's Virginia Standards of Learning test in biology. But the ninth-grader says he takes comfort in knowing that his school, Wakefield High in Arlington County, has scheduled three uninterrupted hours for students to handle the exam.

At other Northern Virginia high schools, the allotted time for the same test is different -- just two hours at T.C. Williams in Alexandria and, until this year, 90 minutes at high-achieving Oakton in Fairfax County.

Johnson, 15, and the biology students at other Virginia schools can take more time if they desire. Officially, the state's achievement tests are not timed, and some students have consumed as much as seven hours to finish one exam.

Still, Johnson said, many teenagers are too embarrassed to take the extra time, even when they need it. He said he will have no trouble with the state algebra test, but because biology is hard for him, anything much less than three hours "would feel like I was in a rush."

As Virginia approaches the first graduation day in which its students might be denied diplomas because of failure to pass Standards of Learning tests, the wide differences between schools in the amount of time reserved for tests are getting heightened attention, as are the increasing amounts of time students are using for each test.

Many U.S. students and their parents are surprised to learn that at least 14 states, including Virginia, let students take as long as they like on achievement tests. This is allowed under the federal No Child Left Behind law, which gives each state great leeway in constructing the tests used to determine which schools will be labeled as needing improvement.

That explains why Wakefield, with 40 percent of its students from low-income families and a 71 percent passing rate on the SOL biology test last year, is giving students plenty of time to go over every question. And why Oakton, with 7 percent low-income students and a 91 percent passing rate in biology, allots far less time.

Maryland and the District still use timed tests. The new High School Assessments in Maryland, for example, are each 90 minutes long, with extra time allowed only for children with disabilities.

Educators say the trend in the Washington area -- as well as nationally -- is toward more time with the tests, rather than less. Oakton expanded its testing period from 90 minutes to two hours this year, after encountering more students who said they needed more time.

Jane Lipp, guidance director at Hayfield Secondary School in Fairfax County, said the number of students taking extended time is rising at her school, too, because the Virginia tests now count for graduation and because of the needs of students with disabilities. About 20 percent to 25 percent of her students are staying beyond the allotted time, she said, particularly in Algebra I and Algebra II.

Some testing experts say these changes are nothing to worry about. "If tests are supposed to be measuring what a student knows, not just how fast she or he can spew back answers, there's no reason for rigid time restrictions," said Robert Schaeffer, public education director for the National Center for Fair & Open Testing, or FairTest.

Some parent groups, already believing that too much emphasis is placed on standardized testing, say the unlimited time robs children of hours that could be used for better things and casts doubt on the test results.

"It's manifestly unfair to have such inconsistent time allotments and procedures, and pretty ironic to boot, considering there are procedures for implementing one-size-fits-all testing," said Roxanne Grossman, a spokeswoman for a parents group in Virginia. Susan Allison, coordinator of Marylanders Against High Stakes Testing, said, "I would be furious if I heard that one of my daughters was squirreled away someplace to stew one extra minute over a standardized test."

Doris Jackson, the principal at Wakefield, said her staff has designed a spring testing schedule that lets students concentrate on the SOL exams with minimum disruption inside and outside the testing rooms. Only students who have exams come to school on test mornings, with the initial testing period from 8:15 to 11:15 a.m.

Students are encouraged to go over their answers, or read quietly from a book not relevant to the exam, until the three hours are up, because no one is allowed to leave the room early -- except for monitored bathroom breaks, Jackson said.

The longest any Wakefield student ever took on an SOL test was seven hours, she said. "It was particularly puzzling because the student was a high achiever who had success with timed tests," and the student passed that test also.

Charlie Ostlund, principal at Oakton High, said the school's 90-minute testing period seemed to work for most students last year, but administrators added a half-hour this year to reduce the number of students who had to be moved to extra-time rooms. One year, he said, there were two students "who milked one test for an entire day, to avoid going to any of their other classes."

Researchers say that letting students, rather than the clock, decide when they are done introduces factors in assessing the results that have not been studied thoroughly. Eva Baker, co-director of the Center for Research, Evaluation, Standards and Student Testing at the University of California-Los Angeles, said adolescent eagerness to get out of the testing room is just one factor. "It is clear that there are multiple incentives for finishing," such as wanting to be with friends or go home, she said, but more research is needed on how students use the extra time when they take it.

A 2003 report by researchers for Harcourt Assessment Inc., the company that produces the SOL tests in Virginia, suggests that time limits for achievement tests based on learning standards might be a thing of the past. In a study of hundreds of thousands of students, the report said, "differences in the average raw scores of students tested under timed and untimed conditions were typically very small. In the majority of cases, the differences amounted to less than one raw score point."

The report said the results appeared to confirm that the time limits the company had been using for exams such as the Stanford 9 achievement test, used in the D.C. schools, were long enough for students to show what they knew. But it also suggested that time limits might no longer be necessary, and the extra time given to students with disabilities might be given to all students without fear that the results will give an inaccurate picture of how much each has learned.

Harcourt Assessment spokesman Mark Slitt said the company's new test, the Stanford 10, can be used as either a timed or an untimed exam.

Stu Singer, the mathematics department chairman at J.E.B. Stuart High School in Fairfax County, said most students finish the SOL tests in the two hours and 10 minutes allotted by his school. But some biology students and many math students stay longer, he said.

"Last year, we had two students test until the late afternoon, 4 p.m., a total test time of over seven hours, and both of them passed the exam," he said. "The teachers at Stuart stress the importance of these tests and encourage students to take whatever time is necessary. Consequently, the story of these two test-takers will no doubt be repeated in future years."