To borrow a line from Mae West, when they're good, they're very good, and when they're bad, they're well below average.

The subject at last week's Prince George's County School Board meeting was not Miss West's "talents" but the scores of 3,748 county public school seniors on last year's Scholastic Aptitude Tests (SATs) and the Admissions Testing Program Achievement Tests (ATs).

Only 294 of the Prince George's students who took the SATs also took the achievement tests, which are designed to measure a higher level of proficiency in several subjects. But their average scores of 558 on the achievement tests continued to be higher than the national average, which was 532 out of a possible 800.

These Prince George's students have widened their lead over the national average from 18 points in 1976 to 26 this year.

On the other hand, the seniors taking the SAT, the broad test of verbal and mathematical reasoning designed to predict performance in college, came in fourth in the metropolitan area derby for the third year in a row.

The average SAT verbal score of 400 in Prince George's was 24 points below the national average and 22 points below the Maryland average. The math score of 446 was 20 and 17 points below the nation and the state, respectively, according to figures recently released by the College Board Admissions Testing Program.

More importantly, the Prince George's SAT scores continued a four-year slide that parallels a 16-year nationwide dip that has been blamed on everything from the effect of television on students' ability to read to the inappropriateness of the test itself.

The test is scored in a range from 200 to 800.

The scores for Fairfax, Arlington and Montgomery counties, Alexandria and the District of Columbia were published early last week.

Prince George's school officials were quick to point out that the national and Maryland figures include scores from private schools, which are, on the whole, more selective in their students than public schools.

Moreover, only 33 percent of the nation's graduating seniors took the test, whose results are useful, and in some cases required, for gaining admission to a large number of colleges.

In Prince George's 43 percent of the seniors took the test, including a larger proportion of academically poorer students who officials say are pulling down the averages.

C. Monica Uhlhorn, administrative assistant for instructional services, said taking the test is pushed by teachers these days, if only because it costs students nothing but the admission fee.

"Years ago you were invited to take the SATs; now students are urged," she said.

She also cited peer pressure as a reason many less-well-prepared students take the test.

"If you don't take the exam, automatically you're identified as noncollege material. If you take the SATs and flunk them, nobody will know because the scores are individually reported," said school spokesman Brian Porter.

Nevertheless, in Montgomery County, 68 percent of the students took the SATs, and scored an average 56 points better on the verbal and 57 points better on the math than Prince George's seniors. On the other hand, in the District, only 29 percent of the seniors took the test and their average scores were 75 and 91 points below Prince George's in the two areas.

"We don't have any embarrassment at all that Montgomery is going up and (we) are going down," said Porter. "That's the way life is."

A brochure from the College Board Admissions Testing Program describes the SAT as "a 2 1/2-hour, multiple-choice test that measures developed verbal and mathematical reasoning abilities . . ."

Scores on the SATs show a relationship to income and race, with students from low-income and minority families making low scores, according to data from the admission testing program.

Prince George's board member Al Golato focused on the above-average scores of the students who took the achievement tests as evidence that the school system works. He said the schools should not be blamed for the "aptitude" of their students, or the lack thereof.

"If you are going to measure the school system, you have to measure achievement. Aptitude is what the students bring to the school system, achievement is what they take out," said Golato.

"We always come down low on the totem pole because we get people on the lowest end of the socioeconomic scale. That damages the image of the school system," he said.

But board member Bonnie Johns emphasized the opposite trends of the top of the senior class versus the middle and the bottom.

"Until we get that middle spread and the lower end of the curve succeeding, we are only doing what tends to happen: the rich get richer and the poor get poorer," she said.

The scores were presented to the board by Elwood Loh, who is in charge of testing and evaluation for the schools. He noted that the SAT is only a predictor of college performance, and not the best one at that.

"The best prediction of success in college is GPA (grade point average) -- there is no question about that," said Loh.