In a time of declining test scores, Fairfax County students' academic achievement is continuing to improve, according to the results of national standardized achievement tests announced yesterday by Fairfax school officials.

The results of the Science Research Associates standardized achievement test series administered by Fairfax schools last fall in grades four, six, eight and 11 show general improvement in reading, writing and arithmetic over the past five years.

The test scores of Fairfax County 11th graders in reading vocabulary was the only academic area to show decline, according to the test results. Other test scores showed stable academic performance and some significant improvement, particularly in the elementary grades.

"The emphasis placed on instruction in basic subjects by both the school board and staff continues to pay off," said Fairfax School Supt. S. John Davis of the Fairfax students' upward trend in academic achievement.

The tests were given to about 8,000 students in the four grades. The test results of individual schools will not be available until later this month or early April, school officials said, but the Virginia statewide results of the standardized tests will be released Thursday.

Last year's Scholastic Aptitude (college board) scores compared yesterday also showed improvement over the 1975 -- 76 school year, while the scores of junior and seniors taking the college entrance tests nationwide dropped two points, Davis said.

Fairfax County six-graders had the highest average score on the Science Research Associates tests, scoring in the 73 percentile in mathemathics, reading and writing combines. The national norm is considered to be the 50th percentile: this means that Fairfax sixth graders scored better than 73 percent of all students nationwide who took the same test.

eleventh graders scored in the 70th percentile, fourth-graders in the 69th percentile and eighth graders in the 67th percentile.

In the elementary grades, fourth-graders improved only three percentage points over last year, but their scores showed an increase of 14 percentage points since school year 1973-74, when they scored in the low 55th percentile.

Sixth-graders also improved three percentage points over last year, but their scores increased seven points since school year 1973-74. Eight and 11th grades' average scores this year are the same as last year and four years ago, according to the test results.

Davis said a drop of six percentage points in reading vocabulary scored by 11th-graders this year will be studied to locate the cause and find a solution. Eleventh-graders scored in the 62nd percentile in reading this year, compared to scores in the 66th percentile five years ago. At the same time, 11th-graders' achievement in spelling has increased to the 55th percentile from the 48th percentile last year.