Arlington elementary students showed improvement on achievement tests for the fourth consecutive year, with some classes ranking in the top 20 percent nationally, Superintendent Larry Cuban reported last week.

"They are scores that any school board would embrace with lust," a beaming Cuban said.

The highest scores among Arlington students taking the Science and Research Associate Tests were made by fourth and sixth graders were in the top 17 percent nationally, while in reading they scored in the top 22 percent. Fourth graders demonstrated similarly high results, doing better than 77 percent of students nationwide in reading, and better than 73 percent in math.

Eighth and eleventh graders, the other two classes taking the tests, did not fare as well. Eleventh graders scored in the 57th percentile nationally in reading, and in the 59th percentile in math. The reading and math scores of the eighth graders put this in the 59th and 68th percentiles, respectively.

The national average of all students taking the test is 50 percent.

Citing a "back-to-basics" program commissioned by the county school board seven years ago, Cuban attributed the improved scores to a commitment to the program and "plain hard work" on the part of the teachers and aministrators.

"This is a clear instance of where persistence and steady hard work by teachers and administrators have paid off," Cuban said. "We're not talking about one or two years, we're talking about nearly a decade."

Test scores in neighboring Alexandria were far lower than those in Arlington. Eighth graders in Alexandria scored higher than 38 percent of all students taking the reading portion of the test, and in the 53rd percentile on the math section. Fourth graders ranked somewhat higher than the national norm, scoring in the 61st percentile in reading and the 57th percentile in math. Alexandria sixth graders did not take the test.

Fairfax County school officials said results of the tests there have not yet been released.

The most marked increase in Arlington scores since the 7-year-old "basics" curriculum began was recorded at the sixth-grade level in math. In 1972, Arlington sixth graders were ranked in the 52nd percentile. In 1980, the sixth-grade class scored in the 83rd percentile.