When 12-year-old Stephanie Kennedy got sick one Saturday morning last April, her mother thought at first that it was intestinal "flu." But when she suggested Stephanie take a shower, her daughter could not get up from the sofa.

"Her eyes looked really funny," recalled Patricia Ann Crabbe, the girl's mother. "Then she looked at me and said, 'Mom, I think I have toxic shock syndrome. Go upstairs and look at the flier in the [tampon] box.' "

Stephanie Kennedy's diagnosis was correct, and her knowledge of the rare, tampon-related illness -- obtained in a health class she had as a sixth-grader the previous year in Crozet, Va. -- may have saved her life, said Dr. Lawrence D'Angelo, who treated the Arlington youngster for toxic shock syndrome at Children's Hospital National Medical Center.

Five years after an epidemic of toxic shock syndrome focused national attention on tampons, Food and Drug Administration officials fear that few girls reaching menstrual age are as aware of the disease as Stephanie Kennedy. Yet, teenagers who use tampons are the group at highest risk for toxic shock.

So the agency announced last week that it is mailing a poster with teaching materials about toxic shock syndrome to all U.S. junior and senior high schools, for optional use in health courses during the next school year.

"Despite alerts on tampon packages, many young women may believe toxic shock has gone away, or is less serious," said FDA Commissioner Frank E. Young. "This educational program will communicate the facts, the symptoms and the need for immediate treatment."

The agency's poster bears the headline, "Toxic Shock Syndrome is so rare you might forget it can happen . . . " On the reverse side are lesson plans, quizzes and information about the illness for teachers wishing to include the subject in health curricula.

Toxic shock is a rare disease, striking only 1 in about 10,000 menstruating women, according to the poster. Although it most frequently affects tampon users, it also can occur in women using menstrual pads, in users of the contraceptive sponge and diaphragm, in non-menstruating women, in people with infections related to insect stings, wounds or surgery, and in men and children.

As of June 1, the Centers for Disease Control had confirmed 2,815 cases of toxic shock syndrome, 122 of them fatal. Of the victims, 2,669 were female. So far this year there have been 52 cases reported, with 4 deaths.

The illness is caused by strains of Staphylococcus aureus bacteria which manufacture a toxin that enters the bloodstream to produce symptoms of fever, vomiting, diarrhea, dizziness and a distinctive sunburn-like rash. Severe cases can progress to shock, a condition in which blood pressure drops low enough to cause organ damage or death if not promptly treated.

Just how tampons contribute to the condition is not fully understood. A Harvard study published last month suggested that the high-absorbency tampons most associated with toxic shock during the 1980 outbreak contained fibers that removed magnesium from vaginal fluids, a situation which apparently promotes manufacture of toxin by the bacteria. Products containing these fibers -- polyester foam and polyacrylate rayon -- have all been removed from the market.

But the Harvard study does not explain cases occurring in users of products that do not contain the magnesium-binding fibers.

Nor is it known why the number of cases of toxic shock occurring in non-menstruating individuals has risen over the last few years, said Dr. Mitchell Cohen of the Centers for Disease Control.

Twenty to 25 percent of cases of the syndrome occurring now are not associated with menstruation, as opposed to about 10 percent in 1980, said Cohen, who is assistant director for medical science at the CDC's division of bacterial diseases. And in 38 percent of non-menstrual cases, the Staphylococcus strains isolated from the victims do not even produce toxin, he said.

"Despite the fact that we've studied the Staphylococcus for over 100 years, we know little about how it causes human disease," he said.

An FDA spokesman said the agency is "leaning toward" requiring labeling on tampon packages that rates the product's absorbency on a numerical scale, much as sunscreens are rated. Some studies have indicated that the higher a tampon's absorbency, the greater the user's risk of getting toxic shock syndrome, so such a rating would enable women to compare products of different manufacturers.

"That's a direction that women have been urging the FDA to move in," the spokesman said.

D'Angelo, chairman of adolescent and young adult medicine at Children's Hospital, said he was delighted to hear of the FDA's educational campaign, because he has the impression, from recent tampon advertisements, that some manufacturers are trying to persuade young girls to use tampons from the time they first start menstruating.

"I think that's a new focus," he said. "There are products, too, appearing on the market. They're smaller, they're slimmer, they're easier to insert."'

He cited an advertisement for Tampax tampons, published this year in Glamour Magazine, which begins, "I taught Karen algebra, she taught me about my first period and Tampax." The speaker signs herself "Susan, Age 14," and later urges the reader to "make Tampax the tampon you use for your first period, and every period."

Another, in this month's Seventeen magazine, describes a new model of Tampax tampons called Petal Soft, featuring a soft plastic applicator. "It's perfect for beginners like us!" says the teenager in the ad.

Paul Konney, vice president and general counsel for Tambrands Inc., makers of Tampax, said "It is true that we do focus some of our effort against younger women who are just beginning to menstruate and have to decide what form of protection to use." But, he said, "there has been no dramatic change in focus."

D'Angelo is particularly concerned about newly menstruating women's use of tampons because they have a higher risk of toxic shock syndrome than women in their 20's or 30's. The reason why younger women are especially susceptible is unclear, but some scientists speculate that they haven't had enough time to develop protective antibodies against this bacteria or its toxin.

D'Angelo said he advises adolescents in his practice not to use tampons until they have been menstruating for at least two years, to change tampons frequently, to avoid high-absorbency tampons, and never to wear tampons overnight.

Stephanie Kennedy did everything the FDA advises young women to do when they suspect they may have toxic shock syndrome. She removed the tampon at the first sign of illness. She sought immediate medical attention. She informed doctors that she was using tampons, so they could do tests to establish the diagnosis.

D'Angelo said that, after Stephanie's recovery from toxic shock syndrome, he wrote to officials at her former school in Crozet, Va., where she had learned about the disease.

"This girl's knowledge of the condition probably saved her life," the letter said. "I wanted to share with you this episode so that you could appreciate the fact that you are indeed providing an excellent service to the students of your school."