Kim Startup first had sex when she was 12 years old, an age when contraception seems a well-kept secret of the adult world.

"When I was 14, I thought, 'Sooner or later it's gonna catch up to me.' I mean, I was beating the odds as it was," she said.

It did catch up with the teen-ager, who two years ago, at age 15, gave birth to a son in Ayer, Mass., after running away from her Falls Church home.

The added responsibilities of a baby often rob teen-aged mothers such as Startup of the energy and time that they need to attend school. But a program for teen-aged mothers at Fairfax County's Groveton High School brought Startup, now 17, back to the classroom. Now she talks about the "fun of making grades" and about going to college.

Groveton is one of an increasing number of schools across the country that have tried to respond to what educators have called the "epidemic" of births to unwed teen-aged mothers by initiating programs that make it easier for them to return to school.

Nationally, though the number of young women in the 13-to-19 age group has declined in recent years, the number of unmarried teen-aged mothers more than tripled between 1955 and 1982, jumping fom 72,800 to 269,346, according to the most recent research from the National Center of Health Statistics.

In the Washington area, Groveton and a small number of other high schools help young mothers to return to school by offering them a place on campus to leave their babies while they attend classes and by providing instruction on child rearing as well.

Other programs are available only while a student is pregnant and do not provide day care. For the teen-ager who cannot find a willing relative to care for her child, often the only option is to drop out of school.

But like many such programs across the country, the Groveton project has few supporters, say those who run it. Parents of pregnant teen-agers, often embarrassed about their child's situation, are not likely to lobby for school programs the way parents of the handicapped or the gifted do. And the programs are not strongly supported by top school administrators.

Fairfax Superintendent William J. Burkholder and School Board Chairman Mary Collier argue that schools should not be in the business of providing day care to infants of teen-aged students. That is a social service that county government should provide, they say.

Said Collier, "We're responsible for educating children, not providing day care for infants," noting that the school system has high school adult-education classes that teen-aged mothers can take in the evenings. "We certainly don't want to be encouraging girls to become pregnant with this kind of program," Collier said.

The result is that the programs have taken on what one Fairfax school official called an "illegitimate" status that makes it hard to attract both sponsors and students the programs are meant to help. Though more than 500 teen-agers had babies last year in Fairfax, only 11 are enrolled in the Groveton program.

Burkholder said he has no plans to scrap the Groveton program but is concerned that it is a "fringe program" that "begins to get into an area where schools shouldn't get involved and can't afford to get involved." He says he would oppose expanding the program if such plans included a larger day care center.

Pam Robinson, one of the two founders of the Groveton program, says the school system abdicates an educational responsibility by playing down programs like Groveton's. The program, she said, "gives teen-age mothers a chance to finish high school and learn how to care for their babies, and it gives the babies an environment that fosters the desire to learn."

It was that philosophy that pushed Robinson and Joan Hartman, a home economics teacher at the high school, to apply for the $10,000 grant from the March of Dimes that launched the program two years ago.

"We'd see pregnant kids in the hallways and wonder why we didn't see them anymore when they had their babies," Hartman said.

Hartman and Robinson, taking advantage of an enrollment decline in home economics classes, founded the program in the school's demonstration kitchen. They furnished the room with playpens, cribs and high chairs, and they converted the large cooking counter to a diapering table.

After six months of negotiations with the state health department, the program opened in spring 1983 with seven teen-aged mothers. A total of 27 have enrolled since from eight Fairfax County high schools, including Groveton, Annandale, Hayfield, Edison, Mount Vernon, Robinson, Lee and Fort Hunt.

At 7:15 each morning, while most students are strolling to school, the young mothers are turning over their babies to the four day care helpers who will feed, clean, and entertain them until the school bell signals the end of classes.

"He's getting smarter," said Startup of her 18-month old son Freddie. "He's more active now than he used to be because he's got someone to play with all the time."

The day-care center is several doors removed from the school's main hallway -- a discreet spot for a program that developed discreetly.

The program has been strongly supported by the school system's home economics supervisor, Margaret Morris, and by Groveton's principals, according to Robinson.

However, it has never been publicly presented to the School Board. Not until this fiscal year did its $35,000 cost, most of which is reimbursed by federal and county agencies, slip into the school budget.

Two-year-old De-Shaun Stokes grabs for the doll his mother used to play with. But his mother Donnetta, 16, is not about to let go.

"It's my baby," Donnetta says, childlike possessiveness stiffening her voice.

Eventually, though, Donnetta turns teasing and tender, and she delivers the toy to her son.

In the past year, she has learned not to treat her baby like an animated doll, say school officials.

"She was like a little girl with a toy, except the toy happened to be alive," recalls Vera Blake, assistant principal at Groveton. "But she's learned since how to care for and nurture her baby."

Stokes' maternal maturity is largely a result, Blake says, of the Groveton program. But her greater accomplishment, says Groveton Principal Paul Douglas, is having stayed in school at all. "All our predictions were that Donnetta would not be here today," he said.

Stokes cannot afford a baby sitter, and her mother, who works, is not home to be one. Facing Stokes every day is a 5:30 a.m. alarm bell that wakes her up in just enough time to wash, dress and calm and carry her baby out the door.

Then, there is the mile that Stokes must walk, with baby and books, to school after she steps off the Metrobus. No school bus transportation is provided for Stokes and other students in the program who live outside of the school's regular attendance boundaries.

"That's probably what gets me so mad, is when I have to go down that hill, and it's so cold, and I have my baby out in it," she said.

Beyond these physical obstacles, Groveton administrators say, Stokes had psychological problems evidenced in outbursts of intolerance toward her child. "She was hurt, bewildered, alone -- except for this baby she didn't know what to do with," Blake said.

A year later, Stokes still talks sometimes in an abrasive tone. And she still seems disturbed by her early motherhood. But Stokes says she is determined to graduate.

"If you don't have an education, you don't have nothing," said Stokes. "I have more reason now. I have a child. And I have to give him the best things in life."

Many of the teen-aged mothers at Groveton say the program for teen-aged mothers has given them the solidarity they need to overcome their fears and social unease.

"Some people would just stare at me like I had a disease or something," said Johanna Lund, 15, who brings her 6-month-old girl to the day care center every day.

That's why, said Stokes, when "you're down and out and you need someone to talk to, it's people here that will talk to you. If another teen-ager has a baby, it makes it easier for her to understand. It's like, if you're pregnant, you'll go to your friend first, and then to your mother."

Still, few students in the county know about the Groveton program. Many mothers in the program say they learned about it through unofficial sources -- a friend, a relative.

"I think we're missing the boat in terms of referrals," said Groveton principal Douglas. "We had an obviously pregnant girl here going to classes since the beginning of the year and no one had bothered to tell her about the program."

On the national level, few school systems have designed special programs to keep teen-aged mothers in school, according to Martha Burt, analyst for the Urban Institute and coauthor of "Public Crisis, Public Cost: Perspective on Teenage Childbearing."

In 1972, Congress passed a law prohibiting schools from expelling students who become pregnant. Six years later, with the the Health, Services and Pregnancy Prevention and Care Act, came the federal government's first program to encourage teen-aged mothers to finish high school and stay off the welfare rolls. The program was also intended to improve the health of teen-aged mothers and their babies, who tend to be underweight and sickly.

In its first year, the federal program had a $740,000 budget that did not directly contribute to any schools. Today it has $13.4 million, which partly funds 71 programs, 11 of which are school-based.

Yet, despite this federal investment, "The overwhelming impression I get is that most schools do nothing and many schools tend to discourage pregnant kids from staying in school," said Burt.