Fairfax County's 90,000 students in grades seven through 12 may see some pluses on their grade cards next fall, but no minuses, under a revised grading system approved by the county school board last week.

The new system also provides that students taking honors and advanced placement courses will receive automatic half-grade increases, or pluses, to compensate for the difficulty of the courses.

The changes in the grading system are designed to enable teachers to better reward academic excellence and to help college-bound students taking advanced courses.

The present system for ranking students in county high schools, considered important by most college admissions offices, gives students with As in physical education, typing or basic math the same rank as students with As in calculus or college-level history courses.

The present five grades -- A, B, C, D and F -- will be expanded to eight by the addition of D+, C+ and B+. Honors and advanced-placement courses will be weighted by a half grade -- for example, a B in a honors courses would automatically be recorded as a B+.

The numerical scale for an A grade will remain the same, 94-100. Other grades will be based on a desending 5-point scale. For example, a score of 90 to 93 will be a B+, and 84 to 89 will be a B. A score of 63 or below will be an F.

The changes were backed by the Fairfax County Council of PTA's which orginally proposed that pluses and minuses be added, creating a 13-step grading system. Several high school PTAs also had recommended changes in the grading scale. One suggestion would have instituted a 10-point range for each grade, with As equalling scores of 90-100, Bs 80-90, etc.

However, the school staff had urged that these grading scales not be adopted because they were significantly different from the existing system. The staff said such systems would require extensive computer and report-card changes and additional staff, and that the changeover would take at least a year.

PTA Council President Toni McMahon praised the school board's action. "I think people will be very pleased with the changes," she said after the meeting.