The proportion of students passing Virginia's high school competency tests rose last spring compared to the previous fall and black students made the most substantial gains, the state education department reported.

Even so, about 34 percent of blacks failed at least one part of the 159-question exam, which covers basic skills in reading and mathematics. The failure rate for whites was about 9 percent.

Under state law, all students will be required to pass both the reading and math sections of the test before they can graduate from high school, starting in 1981.

"Its my feeling that after the fall results were out, people took the tests more seriously and took more time to prepare for them," said Gerald W. Bracey, director of research, evaluation and testing for the Virginia education department.

Some school divisions really didn't do much [to prepare] before the first 71,000 10th graders. About 6,500 10th second ones they made some changes."

Overall, the proportion of students passing both parts of the test was 84.9 percent in April, compared to 82.2 percent who passed both parts when the exam was first given in late October and early November.

For blacks, the failure rate dropped from 42 to 34 percent; for whites it dropped from 11 to 9 percent.

The spring test was given to about 79,000 ninth graders throughout the state. In the fall it was given to about 71,000 tenth graders. About 6,500 10th graders also took the test in the spring, but the state reported no data on how many of them were repeating an exam they had failed and how many were taking the test for the first time. The spring scores for 10th graders were not included in the fall-to-spring comparisions.

In Northern Virginia, the percentage of students passing both spring exams was 94.2 percent in Fairfax County, 89.9 percent in Loudoun County, 88.7 percent in Prince William County, 85.9 percent in Arlington, and 76.9 percent in Alexandria.

Among whites in all these districts, more than 90 percent of the students passed the tests. Among blacks there was greater variation, with the pass rate for both tests ranging from 71 percent in Fairfax to 58 percent in Alexandria.

Donald Dearborn, an assistant superintendent in Alexandria said his school system is making a detailed analysis of the test results "to see where most students fell short."

During the past year, Dearborn said, the Alexandria schools made no changes in their curriculum because of the competency tests, but now "if there is a valid void, we will make appropriate revisions."

Elsewhere in the state, two of the districts showing the sharpest improvements were Prince Edward County in Southside Virginia, and Norfolk, both of which have relatively large black enrollment.

In Prince Edward County, where the schools are slightly more than three-quarters black, the pass rate for all students rose from 72 to 91 percent on the English part of the tests and from 55 to 90 percent in math.

After the low scores for the fall tests were reported, "we decided we just had to call a halt to most everything else, and make sure students learned those competencies." Supt. James M. Anderson said yesterday.

Even shop and physical education teachers were required to spend part of each period teaching particular skills in reading or math, Anderson said, "and the students realized the importance of passing these tests."

We figured, why do the rest of curriculum," he said "when so many haven't learned the basics?"

Jack W. Gravely, executive director of the Virginia NAACP, said his organization still has "very serious questions" about the exam even though "we are not ready to make an indictment that it's racially or culturally biased."

"A competency test for students is not the total answer," Gravely said. "It holds children up for ridicule and punishment. It should be used as a diagnostic tool, not as a punishment tool."

During the coming school year, the exam will be given only to 10th and 11th graders who previously have not passed it.

State Supt. S. John Davis said a new version of the exam will be given to the class of 1983 when its students enter the 10th grade in the fall of 1980.

"I think there should be some modifications," Davis said, "so the test could assist better in remediation. But I don't think there will be any drastic changes."

Davis, who became chief of the state education department Aug. 1 after serving as superintendent in Fairfax County, said his staff is developing a new series of exams that will test specific objectives in reading and math for each grade.

Davis said he feels the state should not require students to pass these tests to be promoted from grade to grade.But he added, "They'll give teachers and parents a good idea whether a young person is prepared to go to the next grade level."