Ninth-grade students in Prince George's County public schools scored higher than ever on the state-required functional reading test last fall, but 15 percent were unable to pass the test that measures such basic skills as reading a subway map.

Eighty-five percent of the 9,400 county ninth-graders who took the multiple-choice competency test passed it, according to a school report released yesterday.

That compares to 80 percent the previous year and 72 percent in 1977 when the testing program began.

Prince George's students last fall lagged behind their affluent neighbors in Montgomery County, where 96 percent of the students who took the test passed, and the overall state score of 89 percent.

Prince George's school officials attributed the improvement to the fact that passage of the 100-question test with a score of 80 or better became a statewide requirement for graduation beginning with the class of 1982.

"At first some teachers and students thought that the test might go away," said Louise F. Waynant, director of instructional service for the county schools. "Now they are taking it much more seriously--it's a combination of effort and attitude."

At the request of the Maryland legislature, state education officials drew up a package of tests in 1977 designed to insure minimum competency in 250 areas.

The Maryland Functional Reading Test was the first of those tests to be administered statewide. Additional examinations in mathematics and writing are being readied for next year and may be added to graduation requirements by 1987.

Unlike standard reading comprehension/vocabulary tests, which are still given to county students, the questions on the functional reading test are taken directly from everyday reading challenges faced by the average person. In the untimed test, which takes most students about 90 minutes to complete, the reading passages are more likely to be from tax form 1040 than from Thoreau.

For example, in one test under the category called "Following Directions," students are given directions for making Uncle Ben's Long Grain Rice that include familiar pictures of measuring cups. The following questions are then asked:

"How much water do you use?"

"How much rice does this recipe make?"

"What kind of rice is this?"

The test material also included passages from newspapers and a Montgomery Ward order form.

In contrast to the 15 percent of Prince George's ninth-graders who failed to make a passing score, only 4 percent failed in Montgomery.

Prince George's school spokesman Brian J. Porter blamed the difference on the high turnover of students in Prince George's and the wide range of income groups there. In 1981, 43 percent of county elementary students changed schools, according to Porter.

Changing schools disrupts learning, school officials said, because it usually takes students time to adjust to a new environment and new curriculum.

"You have to look at the overall population in Prince George's. The fact that other counties have a greater percentage passing is very nice," said Porter, "but there is no doubt that kids have a problem with reading and some of those kids are in our county."

When ninth-graders fail the test, state rules require that they be given remedial help. They must then take the test again, every fall and spring, until they pass it.

Most eventually succeed. By last year, 98.9 percent of the county's high school seniors had passed the test.