School-aged children infected with the AIDS virus should be allowed to attend classes only after a team of doctors, school officials and parents agrees that it is safe, according to new guidelines released yesterday by the National Education Association, the nation's largest teachers union.

The NEA also recommended that children infected with acquired immune deficiency syndrome, or AIDS, not be allowed in school if they show visible open lesions, or if they are believed prone to vomiting, spitting or biting their classmates. But any children excluded from the classroom should be given alternative instruction, the NEA said in its first public statement on the controversial issue.

Teachers infected with AIDS should be allowed to remain in their classrooms only after a similar determination that they are not endangering students or other teachers, according to the NEA's proposed standards.

The NEA guidelines closely follow recommendations published in late August by the Centers for Disease Control, and are the first national standards from any education group dealing specifically with the vexing problem of AIDS in schools. A CDC spokesman said yesterday that only 191 children under age 13 were known to have the disease, of whom 120 have died. Three-quarters of those cases involved infants too young to be in school.

But some isolated though highly publicized cases of both teachers and students with AIDS -- including two teachers who died in Montgomery County -- have led to school boycotts, emergency board meetings and a sense of panic in school districts across the country. School boards in San Diego and Hartford, Conn., voted late Tuesday to join the growing number of districts barring AIDS-infected students from classrooms.

NEA general counsel Robert Chanin said yesterday that the union was trying to strike a balance between totally excluding AIDS-infected children and teachers from classrooms and a blanket policy of allowing them into school.

"We think there's a search going on for guidelines," Chanin said. "This is an item on the agenda of school boards all over the country."

He said the guidelines were intended to give districts a starting point in developing guidelines before cases arise, to prevent superintendents from having to come up with ad hoc "interim" policies. This is what occurred in Montgomery County, where an interim policy now bans anyone with AIDS from classrooms until the school board approves a permanent policy.

Still, the NEA guidelines left open a host of complex questions. For example, recommending that AIDS children excluded from class be given alternative instruction in isolation raises the question of who should teach them. "Is it volunteers? Do you assign someone to do it? Do you get an AIDS-infected teacher to teach an AIDS-infected child? Are we setting up a leper colony?" Chanin asked.

At an AIDS seminar yesterday at the Department of Health and Human Services, Dr. James O. Mason, acting assistant secretary for health, emphasized again that AIDS is spread through intimate contact with bodily fluids, largely through sexual contact or exposure to contaminated needles.

"I want to reemphasize what I have said many times: Americans are not helpless against AIDS," Mason said. "AIDS is not easy to catch. It is not spread by casual contact with infected persons or patients."