Fairfax County PTA and student government organizations endorsed the county school system's plans for expanded AIDS education at a public hearing last night, but critics said the lessons should do more to stress sexual abstinence.

The school board is to vote tomorrow night on a proposal to offer lessons about the disease to seventh and eighth graders for the first time, and to expand the lessons offered in high schools last year. The new lessons would begin next semester. In the seventh and eighth grades, classes for boys and girls would be offered separately and students would need parental permission.

In the ninth grade, all students would be taught about AIDS in a new unit and coeducational health classes, with parental permission not required. Ninth graders now are offered AIDS instruction as part of a general education program that varies from school to school.

To approve the new lessons, the School Board would have to remove its policy that forbids teaching about homosexuality or contraception before high school.

Acquired immune deficiency syndrome, which is transmitted via blood or other body fluids, destroys its victims' immune systems, leaving them vulnerable to cancer or other disease. Most victims in the United States have been homosexuals or intravenous drug users.

Of the 16 people who testified at a public hearing on the proposed lessons last night, only five wholeheartedly endorsed the changes. School Superintendent Robert R. Spillane said afterward he did not hear an "overwhelming condemnation" of the proposed lessons, and said teachers already are urged to stress abstinence.

Spillane and his staff promised some changes in response to critics' complaints. including the removal from one pamphlet of a telephone number for the National Gay Task Force. But Spillane said the proposed lessons are "generally sound."

Supporters of the new lessons included representatives of the Student Advisory Council, the County Council of PTAs, and the American Association of University Women.

"The students need to know the truth as early as possible, and hiding facts or realities from them is no way to teach childrean about AIDS," said student government spokesman Matt Aiello.

The rest of the speakers -- all county parents -- said the curriculum materials should preach abstinence, refrain from describing some sexual practices and emphasize that condoms and spermicides do not offer complete protection from disease.

The lessons "will not work to reduce the spread of AIDS and could be disastrous to the health of our young people," said Jo Ann Gasper, a county parent who also is an adviser to the Reagan administration's undersecretary of education. Gasper, who formerly ran teen-age pregnancy programs at the U.S. Department of Health and Human Services, said she was speaking as a parent and not in any official capacity.