Fairfax County School Superintendent Robert R. Spillane said yesterday that he will not readmit a youngster with AIDS to the public schools because medical authorities cannot assure him that the girl poses no risk to schoolmates.

The mother of the child, a 5-year-old kindergarten student expelled last month for having AIDS, filed suit against the county school system in U.S. District Court in Alexandria yesterday, alleging that Spillane's action violates a federal law barring discrimination against the handicapped.

"This kid will be dead in a few months," Spillane said to reporters after a news conference. "What's the point of the lawyer?"

The superintendent said the child's life would be happier if the situation had been worked out peacefully.

The child's name and medical status have not been released. The life expectancy of a child with AIDS is generally longer than that of an adult and can vary from months to years. It is known that the girl is being treated with AZT, the drug that can slow fatal infections resulting from AIDS in some patients.

The lawsuit is the first such case in the Washington area and the fifth of its kind in the country, according to Kenneth Labowitz, an Alexandria lawyer who represents the child. In the other four cases -- two in Florida, one in California and one in Indiana -- federal courts ordered schools to readmit children with AIDS.

The California case involved a 5-year-old Atascadero boy, Ryan Thomas, who was suspended last year for biting another student. "They must have had a lousy lawyer if they lost it," Spillane said.

Children with AIDS have been quietly integrated into regular classrooms in several school systems, including those in Denton, Md.; Prince William County, and Sarasota, Fla.

The federal Centers for Disease Control recommended two years ago that schools admit children with AIDS to regular classrooms without restriction. Only in the case of youngsters who might bite or otherwise draw blood should school officials consider restrictions, the agency said.

Federal officials "all stand by those guidelines," said Jim Brown, a spokesman for the Health and Human Services Department, which includes the Centers for Disease Control. "We've kept those guidelines current. We know the people in Fairfax will finally come to the same conclusion as the people in Sarasota or Denton" who allowed youngsters with AIDS to attend school.

The Virginia Health Department issued guidelines that are similar to the CDC guidelines, according to Chan Kendrick, director of the Virginia chapter of the American Civil Liberties Union. Kendrick said it was his understanding that the Fairfax County child was not likely to bite or otherwise draw blood and therefore would not need to be restricted.

In barring the girl from school, Fairfax officials have cited a state law that prohibits students with contagious or infectious diseases from attending class. Spillane said the youngster was in and out of school before being expelled, and at one point was absent for more than two weeks.

"If you have a youngster with active AIDS, and the youngster cuts him or herself, which is very likely in an elementary school, then you have a real dangerous situation," Spillane said. "I don't want to expose other youngsters to that."

Spillane said the lawsuit was premature because an official review committee will not meet until next week to recommend a final action to him, although he acknowledged the child almost certainly will not be allowed to return to a regular classroom.

"On the basis of what we already know -- active case of AIDS, active virus in the bodily fluid -- it's unlikely under those circumstances that I could be convinced to allow that youngster into the classroom."

Acquired immune deficiency syndrome, which is transmitted through blood or other bodily fluids, strips victims of their immune systems, leaving them vulnerable to cancers and other fatal diseases. Most of the 729 children with AIDS in the United States are believed to have acquired it at birth from their mothers or from contaminated blood transfusions before the blood supply was strictly screened beginning in 1985. The Fairfax child is believed to have contracted the disease from a blood transfusion.

Spillane said yesterday that the CDC's guidelines are outdated because of new information, such as evidence that AIDS has a longer incubation period than first thought. "Our medical authorities in Fairfax County are more up to speed."

He said a three-member committee -- a school system official, a school lawyer and Dr. Fred Payne, assistant county health director -- will meet Dec. 30 to review the case, hear the parent's evidence and make a recommendation to Spillane. If the superintendent decides to expel the student permanently, the School Board must approve the action.

The county Council of PTAs has urged Spillane to adopt a formal policy on AIDS, as other school systems in the area have done. The superintendent refused, saying each case would be handled individually, but that he would follow a procedure of setting up a review committee in each situation.

Spillane said this is the first case in which a child with AIDS is seeking to attend school in the county.