The edge that Northern Virginia students typically have shown over students elsewhere in the state has begun to erode, according to test results announced yesterday.

"Northern Virginia tends to score high on all tests," said Gerald W. Bracey, state director of research, evaluation and testing, as officials announced that all but 6 percent of the state's 10th-grade students who took required competency tests in math and reading last March passed them.

Four Northern Virginia county school systems exceeded the statewide passing rate of 94 percent: Loudoun, with 99 percent; Fairfax, with 97, Prince William, with 96, and Arlington, with 95.

Other area scores were Alexandria, with 91 percent, and Falls Church, with 90.

Bracey said the improvement by the downstate students was more a reflection of improvement by the schools outside the Washington suburbs than a decrease in achievement by schools in Northern Virginia.

"When this test came in, it was not that much of an academic challenge," he said. "Some of the other districts were caught off guard and had to do some curriculum changes and remediation to catch up. The state as a whole now is very close. It's not that big a difference between the state average and what they're scoring in Northern Virginia."

Officials said the results showed little overall change from recent years, although black students in the state continued to close the gap between them and whites and other minorities.

Blacks have shown considerable improvement since 1981, when 12th graders were required to pass the tests in order to graduate from high school, Bracey said. That year 68 percent of blacks taking the tests passed, Bracey said. This year slightly more than 87 percent passed, compared with almost 97 percent of white students and more than 89 percent of other minorities.

Those test scores represent an overall increase of 0.6 percent by white students, 4.4 percent for blacks, and 2.2 percent for other minorities.

"The only thing that would surprise me would be a sudden decline, if something went wrong. But it hasn't," Bracey said.

"Basically what this means is that the kids who took the tests in basic reading and mathematical skills required for graduation are able to pass by the time they get to the 10th grade," he said.

The tests measure the ability to perform such practical skills as balancing a checkbook and understanding safety labels on medicine bottles.

In the Washington area, all the percentages of students passing both tests increased except for Falls Church, which fell from 97 percent last year to 90 percent this year. The percentages there may be somewhat misleading because the system is the smallest in the state and only 78 students were tested there last March.