Northern Virginia students continued to lead the state this year in achievement test scores, even though the rankings of some of the area's elementary school scores slipped along with the state rankings.

Arlington and Fairfax counties ranked among the top three school systems in the state in reading and mathematics scores at most of the grade levels tested, according to results released yesterday by the Virginia State Department of Education. Alexandria and Manassas Park scores ranked the lowest in Northern Virginia, but fell only slightly below the statewide averages.

Statewide, elementary score rankings in reading and math dropped, while secondary school rankings improved. Virginia students ranked above the national average on the standardized test in all areas except fourth grade reading.

"I'm encouraged, but not satisfied," said state Superintendent S. John Davis.

Nationwide, elementary students have shown improvements over the last decade, while high school achievement has dropped, said Donald S. Sale, state testing supervisor. This year's test results showed that Virginia high school students have maintained higher achievement scores compared to the rest of the nation, while elementary students are progressing at about the same rate as the national average, Sale said.

Statewide, individual county's rankings of average test scores varied by as much as 70 points. One of the highest rankings was the 83rd percentile scored by Fairfax County eighth graders in math. That means the students performed in the top 17 percent of students in the nation on that section of the test. One of the lowest scores was the 13th percentile math ranking of 11th graders in Sussex and King and Queen counties.

In Alexandria, where rankings had been edging closer to Arlington and Fairfax averages over the past few years, students this year scored about 20 points below those in the neighboring districts. Fauquier and Loudoun counties continued to rank closer to the front-runners.

In the Falls Church school system--the smallest in the state--test scores were among the highest in Virginia. Eighth and 11th grade students scored in the top 25 percent of the nation in math.

The state used a new version of the standardized Scientific Research Associates (SRA) test this year, making comparisons to past years' scores difficult, state education officials said. But officials still drew these conclusions from the results:

* Fourth-grade reading scores dropped slightly relative to the nation, but rankings in math, language arts, social studies and science increased.

* Eighth graders showed some improvement in all areas, with the most in math, social studies and science.

* Eleventh-grade reading and language arts rankings remained the same, while social studies dipped and some gains were recorded in math and science.