Concerned that tougher academic requirements are forcing high school students to sacrifice elective courses, the Fairfax County School Board is moving toward adding a seventh class period to the school day.

At a meeting Tuesday night, the School Board's Instructional Committee ordered the school staff to outline the options for adding a period to the six-period day at the county's 21 regular high schools after hearing a report from the staff on lengthening the school day.

The additional period, which could begin as early as next year, could cost from $8.2 million to $19.2 million a year, depending on how it is done, according to the staff report. The lower cost would entail shortening the 50-minute periods to 45 minutes and paying teachers for an additional 20 to 30 minutes. The higher cost would involve hiring more than 500 new teachers.

If the board approves the additional period, Fairfax will be the first school system in the Washington area to lengthen the academic day in more than a limited number of schools, according to the school staff. The Thomas Jefferson High School for Science and Technology, a magnet school, already has nine class periods.

"It's something we believe the community, particularly the parents, are anxious for us to institute in order for the children to have a well-rounded education," School Board Chairman Mary E. Collier said yesterday.

Depending on how it is implemented, the idea could face opposition from students and from teachers concerned about the additional workload.

"This is the wrong time for it," said Mimi Dash, president of the Fairfax Education Association, the county's major teacher union. "You can't keep piling on top of people without taking something off the pile."

Many teachers complain that the school system's new merit pay plan, which teachers agreed to in return for a 30 percent pay increase over three years, has imposed additional work. To counter those complaints, the board also is considering lightening the workload of secondary school teachers.

The School Board has been kicking around the idea of a longer school day for four years, since the state raised the number of credits required for graduation from 18 to 20, effective with the class of 1988, and allowed schools to offer an optional 22-credit academic diploma.

The added academic requirements raised the issue of whether students were sacrificing art, music and other electives in order to take academic classes. Three years ago, high schools were permitted to use existing staff members and budget to offer an optional extra period; this year, 14 schools enrolled 364 students in extra-period classes. A study at one high school last year showed that half of the students took electives, in many cases business courses.

The proposed seventh period would be mandatory for all students, unless special exceptions were granted. In addition to permitting students to take elective courses, the seven-period day would help students who failed a class to graduate on time by making it up, would allow time for extracurricular activities without forcing students to stay late and would allow more flexibility in scheduling, supporters say.

The drawbacks, according to the school staff report to the board, are increased costs and additional work for teachers and guidance counselors. Collier said the board also was concerned that the extra period may increase academic competition between students, and was considering requiring that the additional time be used for electives.