Arlington County public school students ranked well above national averages on standardized academic achievement tests last year, but blacks and those with limited English-speaking ability continue to lag far behind whites on those tests, the county school system said yesterday.

In an academic performance report for the 1982-1983 school year, school officials also reported that for the first time all Arlington high school seniors passed state minimum competency tests last year.

The achievement scores show no significant change from those of the previous school year, according to assessment director John Crowder, who prepared the report.

The report compares the results of two tests from the Science Research Associates test series. One measures "educational ability," or students' potential, while an "achievement" test measures what actually has been learned. The two tests, covering reading, mathematics and language, were administered last spring to second, fourth, sixth, eighth and 11th-grade students.

According to the report, the scores of both black and white students were within "levels of reasonable expectation when compared to ability as measured by these tests."

The test results measure student performance against a national average, computed in percentiles. The national average is at the 50th percentile.

Overall, Arlington students' achievement scores ranged from the 57th to the 86th percentiles--meaning that their performance either matched or exceeded that of 57 percent to 86 percent of students nationwide. But black students scored as much as 40 points below whites.

"The school system in general has to be concerned about the gap. It is a significant gap," community relations director Daniel Brown said yesterday. "Some feel encouraged because the gap has closed some. But from my perspective, the gap surely has not closed enough."

Black students' best showing was among second-graders, who ranked in the 64th percentile in composite achievement scores for all three subjects tested. Their weakest composite scores were among 11th-graders, who scored in the 30th percentile, well below the national average.

The weakest composite scores for both black and white students were at the 11th grade, where they ranked in the 63rd percentile. As a group, Arlington's white 11th-graders scored in the 77th percentile.

Among all students, second-graders also had the highest composite scores, placing them in the 86th percentile. White students in this group ranked in the 91st percentile.

The report also compares students with limited English-speaking ability, mostly Asian and Hispanic pupils enrolled in special language classes. Those students generally ranked below the national average.

Here, too, the worst showing was among 11th-graders, whose composite scores placed them in the 11th percentile. Second-graders scored highest, in the 66th percentile.

Hispanic and Asian students without language problems generally ranked below whites and above blacks in most areas, the report said.