Getting an A in a Fairfax County school soon may have a different meaning, if several high school PTAs having their way.

Toni McMahon, president of the Fairfax County Council of PTAs, told the county school board last week that the present grading system "fails the prime test of uniformly rewarding superior academic performance with higher grades and a higher class standing."

Some high school PTA representatives told the board that class standing, a major factor in college admissions, is not a good indicator of ability in Fairfax schools because the existing letter grades cover such a wide range of performance.

Grades for widely different courses are given equal weight in determining class standing.For example, students getting As in gym, typing or basic math are ranked the same as those getting As in college-level history or calculus.

The County Council of PTAs urged the school board to add pluses and minuses to letter grades, creating 13 grades instead of the current five of A,B,C,D and F.

Several high school PTAs want the school board to give more weight to grades in advanced-placement courses, for instance, or change the numerical range a letter grade would cover.

Virginia Caress of the J.E.B. Stuart High School PTSA said that group also supports "expanding the numerical scale for letter grades, perhaps all 90s equalling an A, all 80s a B, 70s a C, etc."

In Fairfax County, an A has a numerical equivalent of 94-100. In other school systems, an A is equivalent to 95-100.

The school board has set no date to consider the grading issue.

In other business, the school staff told board members that some progress was being made on its goals of hiring more minorities and men for teaching jobs and more women for administrative posts.

Last fall the board set goals that by 1983, 11 percent of the county's 6,535 teachers would be minorities and 12 percent would be men. In addition, it proposed that by 1983 45 percent of the supervisory jobs be held by women.

During the last school year, about 19 percent of the 457 new teachers were minorities, bringing the total number of minority teachers to 534, or 8.2 percent of the teaching staff. Last year, minorities made up 7.2 percent of the teaching staff.

However, the percentage of men in teaching positions declined from 11.2 last year to 9.7 percent this year.

In administrative posts this year, 243, or 39.1 percent of the 621 jobs are filled by women, compared to 37.8 percent last year.