Fairfax students have shown improvement over last year in tests measuring them against the rest of the nation's students in such skills as reading, spelling and math computation.

The improvement, which was most evident in the elementary grades, is a result of emphasis on these basic skills, and Fairfax County School Supt. S. John Davis when he released the scores this week.

"We are now seeing the results of the teachers taking up - over the past two to three years - the message of the school board which had designated instruction in the basic skills as a priority," Davis said.

In the fourth grade, for example, Fairfax students rose from the 55th percentile in 1973 up to the 62nd last year and then to the 66th this year. And in the sixth grade, children got higher scores in math computation, going from the 52nd percentile in 1973 to the 61st last year and finally to the 65th this year. A 65th percentile score, for example, means that 65 per cent of the students taking the test nationwide scored lower, and 35 per cent scored higher.

Overall, the median of Fairfax County students' test scores was at the 70th percentile, well above the national average or norm of the 50th percentile, meaning that Fairfax students did better on the average than 70 per cent of all students nationwide who took the tests.

Davis said he hoped the improving scores would muffle critics of education who say that costs continue to go up while test scores go down - "That is just not happening in Fairfax County," Davis said.

Fairfax students did not score well in every area, however. When it comes to spelling, Fairfax County 11th graders are in the 48th percentile, which means that they are worse spellers than 52 per cent of the nation's 11th graders.

These specific test results, Davis emphasized, are used to determine where student achievement is weak so that extra teachers' aides and reading teachers, for example, can be placed at those schools. "I use the (test) information as one factor in determining weak areas where corrective emphasis should be placed," the superintendent said.

And because of the zero-based budgeting procedure introduced into the school system this year, in which limited resources must be a reallocated instead of merely expanded, Davis said the test results will assist the school administration in reassigning these limited resources such as extra reading teachers.

Davis cautioned parents against considering the tests' scores as a final assessment of their children's achievement or as a definitive evaluation of their local school. Rather, he said, they should be viewed as one relative measure of performance only.

"It should be understood," Davis said, "that performance on achievement tests is a result of both in-school and out-of-school factors, and we frequently find that students scoring low are doing well considering their lack of out-of-school advantages."

Davis did say, however, he thought that the battery of standardized tests were a "fair" way - "since we don't have anything better" - on measuring childrens" achievement as well as evaluating the school system, the local schools and even the teachers.

The standardized tests were given to fourth, sixth, eighth and eleventh graders in Fairfax county last fall as required by Virginia's Department of Education. They are made up and corrected by Science Research Associates (SRA) Assessment Survey which is approved by the state.

Results of the tests can be viewed at the school system's central and area offices, at each county school and at each branch of the Fairfax County Public Library. The statewide test score report comparing scores achieved by students in all school divisions will be released in April by the state department of education, school officials said.