Arlington County public school students scored at least 15 points above the national average on standardized reading and math tests this year, although the average scores of fourth, sixth and 11th graders dropped slightly from last year, officials said yesterday.

The scores of second graders generally remained stable, while those of eighth graders rose one percentile in most testing categories.

"The general pattern on the Science Research Associates tests represents exceptional performance by Arlington students," Superintendent Arthur W. Gosling said. "Sustaining test performance at these exceptionally high levels attests to the outstanding work of our students and teachers."

School officials said there is no general explanation for changes of one or two percentiles in scores from year to year. "The simplest explanation is that you have different students in different years. What you're comparing is last year's sixth graders to this year's sixth graders," said program assessment specialist John L. Crowder.

The overall scores on reading, math and language arts tests showed second graders scoring in the 89th percentile nationwide; fourth graders in the 81st percentile; sixth graders in the 80th percentile; eighth graders in the 76th percentile and 11th graders in the 70th percentile, meaning they scored better than 70 percent of all students in the nation who took the test.

The Virginia Department of Education requires fourth, eighth and 11th grade students to take ability and skills tests in reading, math, language arts, use of reference materials, social studies and science. In addition, 11th graders take a test in applied skills.

Arlington also tests second and sixth graders and this year tested kindergarten students for the first time.

Eleventh grade students, whose average scores were the lowest of all grades, ranked slightly lower or the same as last year in all test categories, after showing gains last year of two to six percentiles.

Crowder said the average scores of students in higher grades should improve as more rigorous graduation requirements and recent curriculum changes take effect on younger pupils.

Fairfax County and Falls Church released their test scores on Wednesday and students in both jurisdictions scored well above the national averages.