Tenth graders in Prince George's County public schools scored at or above the national average in basic skills testing this year for the seventh consecutive year, the county school board was told last night.

The results of Comprehensive Tests of Basic Skills, given in March to 7,905 10th grade students in the county's 20 public high schools, show that students overall improved slightly over last year. Prince George's students placed in the 56th percentile of all students tested nationally, slightly above last year.

However, scores indicate that on average, students are still weaker in math than language, schools spokesman Brian J. Porter said.

In the test report issued yesterday, vocabulary scores for Prince George's students indicated that their skills in March were at the 10th grade, fifth-month level, one month ahead of the national average. In punctuation and other language mechanics, Prince George's students scored on average at 11th grade, eighth month level, compared with the national average of ninth grade, seventh month.

Average math computation skills among 10th graders were at the 10th grade, eighth-month level -- four months above the national level, but "math concept" scores -- an additional test of math comprehension -- were two months below the national average.

A school-by-school breakdown showed that scores at Potomac and Parkdale high schools in particular showed "substantial gains" over those of last year, the report said.

The overall performance at all schools "shows that our school system is improving," Porter said.

The school board began requiring that 10th graders take the exam about 10 years ago as a means of assessing progress and class curriculum. The 10th grade tests are given in addition to the state-required California Achievement Test for grades three, five and eight and the Maryland Functional Tests that all high school students must pass in order to graduate.

Some county educators and board members disagree about the necessity of the 10th grade testing. Elwood Loh, supervisor of evaluation and research and the county school official in charge of testing, says that while he favors the concept, "the test itself is out of date."

Loh said he wants to find a different type of exam that will better help teachers pinpoint students' weaknesses.

Yesterday Porter said that while school superintendent John A. Murphy is pleased with the latest results, he wants to see all test scores improve.

"One of his goals for this test and all tests is to move into the 75th percentile," Porter said.

In other action, the school board decided to ask the County Council for an additional $665,000 for its magnet school plan intended to improve programs at predominantly black schools.