Prince George's public school students performed about the same on standardized tests in math, vocabulary and reading ability in 1980 as they did in 1979, according to report released last week by county school officials.

The sampling of third, fifth, seventh and ninth graders who took the tests, calls the Iowa Test of Basic Skills, continued for the third straight year to score slightly below available Maryland averages and as much as a year below grade level in some skill categories. Based on a sampling made at the time Iowa test was developed, these grade levels also continued to perform below the national "norms."

The county's 10th graders, on the other hand, scored at or above national averages and close to grade level on the Comprehensive Test of Basic Skills, which was administered only to 10th graders.

"We're not really happy with (the test scores)," said Dr. Elwood L. Loh, who supervised the testing of the 38,000 students last spring, "but we're glad that the trend was not negative."

School officials blame the Iowa test, which they believe is outmoded, for the discepancy between the 10th and other grades' performances. The Iowa test were first developed in 1968.

Officials also said the high turnover of students -- an average of 40 percent in each classroom -- was an important factor in the poor performance of the lower grades. Last year the county public schools took in 14,000 new students above the grade of kindergarten, according to spokesman Brian Porter.

"Kids who have not been in our elementary schools do not score as well," said Loh.

Dr. Louise Waynant, director of instruction for the schools, said that the high turnover rate, particularly in the early grades, disrupts instruction in the basic reading and math skills critical to a child's future performance in school.

School officials say the turnover is due to high mobility within the county as well as new students from outside the county, but they could not say whether the problem is unique to Prince George' schools.

County schools will spend close to $1.6 million over the next three years on new standardized texts to close the gaps caused when parents move children from one school to another. The new books are part of a "back to basics" emphasis begun three years ago.

The systemwide averages conceal wide variations in the performances at individual schools. For example. Bowie High School 10th graders averaged almost one year above grade in vocabulary and reading comprehension and two years above grade in math skills. Their counterparts at Fairmont, on the other hand, were as much as two years behind in those areas.

The Iowa test also included a nonverbal Cognitive Abilities Test, comparable to the widely known IQ test. In several elementary schools, such as Kenmoor in Landover and Mary McLeod Bethune in Chapel Oaks, students with average and above average "intelligence" scored well below expectations.

School officials said the purpose of the testing program is to spot such discrepancies, which may be an indication that teachers in some schools are not pushing students to their full potential.

"We're going to make sure that those kids get the kind of instruction they are capable of taking," said Loh. "Their ability scores indicate they could do much, much more," he added.

The Iowa tests will be replaced this year by the newer California Achievement Tests. The State Department of Education feels it will be a better fit for the county's cirriculum. The Iowa tests also present statistical problems in interpretation for seventh and ninth graders, according to Loh.

The new test is already under fire from school board member Norman H. Saunders of Camp Springs, who believes it may not be appropriate for Maryland students.

The California test, developed by a division of the McGraw-Hill Publishing Company, has nothing to do with the West Coast state, other than the location of its offices, said Loh.