Prince George's County public school students who took standardized achievement tests last spring scored below the national norms in most cases.

The results of the Iowa Test of Basic Skills given to 39,978 third, fifth, seventh and ninth grade students showed that students did best in language skills and worst in mathematical skills. The results are similar to those attained by Prince George's students in past years.

The test, which is designed to measure academic achievement in vocabulary, reading, spelling, punctuation, problem-solving and other areas, is given each year to students in all public schools in Maryland.

Prince George's and other county school systems use the scores to determine which subjects need to be emphasized to a greater degree in the classroom.

The scores released this week by the school board show that third graders tested at or above the national norms in reading comprehension and language skills, but below the norms in vocabulary and mathematical skills.

Fifth graders tested at the norms only in language skills. Seventh and ninth graders tested substantially below the norms in all categories of the test.

Victor Rice, head of testing and evaluation for the county schools, said yesterday that seventh and ninth grade scores appear low when compared to the national norms because the norms are inaccurately high.

"Those norms were developed in 1970 and you can see the same discrepancy throughout the state. The entire state of Maryland cannot be that different from the national norms," he said.

On a second achievement test given last spring to 9,597 tenth graders, scores were still mostly below the national norm but the gap between the county scores and the norms was much smaller than that for seventh and ninth graders of the Iowa test, Rice said.

Gus Crenson, sopkesman for the state Board of Education, said yesterday that the Iowa test will no longer be used to measure the progress of public school students because of the problems in the norms it uses.The test has been used for six years.

"We have seen the trend statewide. The problem is that the Iowa was developed nine years ago and no longer reflects our schools' curriculum. The new test (California Achievement Test) was considered a better match for what we teach," said Crenson.