The Fairfax County School Board approved expanded lessons about AIDS last night for seventh through 12th graders after dropping some references that critics said encouraged teen-agers to have sex.

The lessons, to begin next semester, will teach seventh and eighth graders about acquired immune deficiency syndrome for the first time. Lessons for high school students began during the last school year. The vote was 9 to 0, with board Chairman Mary E. Collier abstaining after her motion to play down mention of condoms and spermicides died for lack of a second.

The county's AIDS lessons will emphasize that abstinence from sexual activity and intravenous drug use is the best way to avoid getting AIDS, but that condoms and spermicides should be used by those who are sexually active.

Seventh and eighth grade classes will require specific parental permission, and boys and girls will be given separate lessons. Beginning in ninth grade, classes will be coeducational, and although a parental permission slip would not be required, parents could remove their children from the ninth and 10th grade classes.

AIDS, which is transmitted by blood or other bodily fluids, destroys the body's immune systems, leaving it vulnerable to cancer or other fatal diseases.

Of 23 people who spoke at public hearings Tuesday and last night, eight fully supported the lessons. They included representatives of the county Council of PTAs, Student Advisory Council and American Association of University Women. School officials said most of the 200 parents who saw the films and written materials generally approved.

But critics, including a dozen county parents, protested that the lessons would not send a strong enough message on behalf of abstinence, and objected to mentions of condoms and spermicides.

The board responded by dropping some curriculum materials and editing lines from several films, such as an AIDS patient's statement that "you need to practice safe sex and be careful who you are with."

The board retained curriculum goals stating that condoms and spermicides be discussed beginning in seventh grade as a way to reduce the risk of sexual activity. "We listen and we take into account what the community says," said board Vice Chairman Joy Korologos. She said the central message should be that "there is no safe sex."

But critics on both sides protested the newly approved lessons. "We're not getting a strong abstinence message," said Jo Ann Gasper, a county parent who also is a high-ranking official in the U.S. Education Department. She said the school system should not "offer the safe sex message" by mentioning condoms and spermicides, but should leave that to outside groups.

Jason Hintz, the nonvoting student representative on the School Board, offered the opposite view, saying students may "turn off" the lessons because the films have been edited. "Students recognize when information is not given to them," he said. The proposal approved last night also drops the board policy prohibiting discussion of contraceptives and homosexuality in intermediate schools. It also provides for training of fifth and sixth grade teachers to answer student questions.