In one Northern Virginia high school this year, 53 teen-agers will become mothers. The girls are students at T.C. Williams High School in Alexandria, and unlike their counterparts in other generations, all have decided to keep their babies, according to principal Robert Hanley.

But more importantly, at least for school officials, all 53 have decided to stay in school.

"Ten years ago, I would patrol the halls looking for girls who were pregnant. If I heard a rumor that a student might be pregnant, I would immediately tell the student she had to leave. It didn't matter whether she was showing or not. Those were the rules," said Hanley, as he sat outside a conference room where a weekly teen-age parenting class takes place. "Now if we hear a girl is pregnant and wants to keep the baby, we encourage her to stay in school as long as she feels itis physically possible.

"We're not necessarily seeing a large rise in the number of pregnant girls. What we are seeing, however, is a tremendous rise in the number of girls who stay in school during the pregnancy and then return after the birth."

T.C. Williams is the only public high school in Alexandria, and out of 1,200 female students in grades 10 through 12, officials estimate that more than 70 are pregnant. In addition to the 53 students still in school, school nurse Mary Ann Heil estimates that at least another 20 students may have decided to continue their pregnancies while trying to keep them a secret or dropping out of school.

"Now that spring is here and the layers begin to peel off, we begin to see more and more tummies," Heil says.

Still, Hanley and others involved in the special parenting program are proud of the increasing number of students staying in school. Although officials have no figures on the number of pregnant teen-agers who stayed in school before the program started, they say that last year, the first year of the special program, 45 students remained in school and they hope more students will take advantage of the program.

"I'm sad that (these pregnancies) happen," Hanley says. "I don't think the girls realize what they're getting into, but we're not here to pass judgment upon these girls. We're trying to help them."

All 53 students attend regular classes, and all are encouraged to enroll in the special parenting class. This year, 37 of the 53 students are in the class.

The major purpose of the parenting class is to help these young girls, often as much children as the babies they are about to bear, face the realities and responsibilities of parenthood. A confident parent, experts say, is a good parent -- and that is the goal of the parenting class.

". . . One of the most important things we can do for them is to give them a good self image," Hanley says. "We want them to be proud of their babies and not to hide them. Things will be tough enough without the additional element of shame."

During the last decade, births to teen-age mothers have declined at the same time teen-age pregnancies shot upward, according to a study by the Alan Guttmacher Institute, an independent research firm. One reason for the decline in births, according to the institute, has been the increasing availability of abortion. Still, the institute reports, four out of every 10 girls will become pregnant during their teen-age years.

At T.C. Williams, many of the 53 students say the father of their child was the first and only boy with whom they had sex.

"You're not talking about promiscuous kids," says nurse Heil, who refers pregnant teen-agers to the parenting class. "Promiscuous kids don't get pregnant because they take precautions. These are kids who got caught the first or second time around."

Instead, educators say, the story of young motherhood at T.C. Williams is the story of a school system and a society becoming more accepting of teen-age pregnancy. Part of that acceptance has come from a federal ruling that requires public school systems to allow pregnant teen-agers to remain in school. In addition, Alexandria adopted a policy in 1976 that requires all students to remain in school until the compulsory education age of 17 unless they have a medical excuse to be out of school.

In Northern Virginia, Arlington County is the only other school system with a special program for pregnant teen-agers. In Arlingtion, officials say 36 students, out of about 3,500 high school girls, are enrolled in an off-campus education center for pregnant teen-agers. In Falls Church, where there is no special program, two teen-agers out of about 200 high school girls have remained in school during their pregnancy this year. And in Fairfax County, the nation's 11th largest school district, with more than 21,000 girls in high school, officials say they have no idea how many pregnant students remain in school. Fairfax has no speical classes or counseling for pregnant students, and officials say they are studying the problem.

State public health figures reveal that in 1979 in Fairfax County, the last year for which figures were available, more than 500 teen-agers gave birth, while another 1,200 became pregnant.

At T.C. Williams, school officials attribute the comparatively large number of pregnant girls attending class to a number of factors. Primary among them is acceptance by peers and administrators.

"At my other school (in Oxon Hill), there was hardly any pregnant girls. Here, it doesn't seem to matter . . . Nobody ever says anything to me about it, or if they do it's to ask how I am or if the baby has kicked," says 17-year-old Lori Cookson, who decided to stay in school because she believed she needed to finish her education and would feel comfortable at T.C. Williams. Cookson's baby is due Sept. 2, one day before her 18th birthday.

Another major factor, others say, is the support of the black community. Forty-nine of the 53 pregnant teen-agers at T.C. Williams are black, in a school system where blacks make up 40 percent of the enrollment.

"I feel sorry for some of my white friends who got pregnant," says one high school senior with a 2-year-old child. The student, who was referred to a reporter by the school nurse, asked that her name not be used because her relationship with the child's father is strained. "It's not that white people don't care, it's just that they've got family going against them.

"Black people are used to hustling for their children."

Recently, three T.C. Williams students agreed to discuss their pregnancies and motherhood with a reporter. The students were referred to the reporter by school nurse Heil and principal Hanley, and agreed to the interviews after discussions with either Heil or Hanley.

Two of the girls, 17-year-old Lori Cookson and 18-year-old Patricia Lewis, are in the special program at T.C. Williams. The third, who has a 2-year-old son, is not in the program but is a senior at the school.

"When my mother found out Mark and I were having sex, I promised I would go and get some form of birth control. I did go to the clinic, but I never went back. I was afraid when they told me about the examination," Lori Cookson, a senior at T.C. Williams says quietly as her 11-year-old brother wandered in and out of her bedroom.

"I made a mistake and suddenly, I have a lot more decisions to make. Everything we wanted to happen in two or three years has now been condensed into one summer: careers, jobs, marriage."

Cookson looks nervously around her tiny bedroom filled with brightly decorated cards, Victorian candy boxes and stuffed animals -- all gifts from her fiance, who is her child's father. Cookson and her fiance, a reservist at Fort Belvoir, plan to marry this summer after the birth of the baby. Cookson is afraid if she marries before the child is born, her delivery expenses will not be covered by her father's medical insurance.

"I guess I'm happy about it now. I have to be. In a way, though, I wish it could have happened later. There is so much I wanted to do."

Still, Cookson never considered abortion or adoption.

"When you really love someone how can you give up your baby?" she asks.

Cookson and the boy met more than a year ago when she was working as a cashier at the Hardee's up the street from the row of identical brick houses where they both lived. They started talking one day, she recalls, and she knew the feelings "were just sort of there. I know he was the right one."

He was also the first and only boy with whom she had sex. And even though she had promised herself that she would never get married or have a baby until she had a career, as an accountant perhaps, she also says that "it's right to have this baby. I wouldn't be pregnant or have had sex if I wasn't in love." Night school, instead of regular college, will have to come later.

Cookson's mother, Louann, listens as her child talks, ever assuring and ever supporting. She is worried that young motherhood and marriage might be rough on her daughter and she hopes that the dreams Lori has of a house, a happy baby and husband won't be shattered. But she also knows that her daughter has decided to face up to the responsibilities that will come with the baby.

"It's is her life and the decisions are hers to make," Louann Cookson says. "We will help her every way we can. But it's tough enough taking care of yourself when you're young. I can't imagine also having a baby."

A scream splinters the noon air before the door swings open and a young child tumbles into the apartment. His mother follows, toting the first of many loads of Saturday washing. The child is two years old. His mother, although not one of this year's 53 pregnant students, is a senior at T.C. Williams.

During most free moments, like this one, the two are together. They sleep in the same queen-sized bed. They go to the park together. Today, they will sit side-by-side in barber shop chairs when the 2-year-old gets his haircut.

"Outside of school, I never go anywhere I can't take him," the young mother says proudly. "It's me and him."

The petite girl, whom a school nurse describes as calm and confident, bends the limbs of a green toy robot nervously back and forth as she asks that her name not be used. She rarely sees the child's father anymore and she does not want him to read about her. She is dating another man now and is happy. But she already has cautioned the new man, "If he can be crazy about me, he can also be crazy about (my son)."

The young woman rarely rises from the brown-and-white fake-fur sofa, but her eyes follow her child's every move. Questions are answered only after her son is appeased. Her son, the young woman explained again and again in different variations, is her life.

Those are telling words from a young woman whose life has never been easy. Her mother died when she was 10, leaving her father with 10 children. Later, when she was 14, she moved from her home to southern virginia to live with her brother in Alexandria.

"In a way, I never really had that much fun before I got pregnant. I would stay in the house and clean a lot. I never liked discos and didn't know how to party. I didn't really have any place to go.

"I didn't have anything that was my own . . . but now I have (my son). I can stay here and play with him or take him out to play with his cousins. We go to the park. We go shopping.

"He has brought joy to my life."

Soon, she says, she would like to move out of her brother's apartment and find her own place. She would like to be a beautician or join the Army, but with a 2-year-old she knows her youthful hopes cannot always be fulfilled.

"Being a mother brought a lot of responsbilities. I didn't think about. I got a lot of help from my family and friends, but still I know that in the end, (my son) can only depend on me.

"I'd like to get out and find out what life is all about. But it's hard sometimes. If someone told me they wanted to get pregnant and have a baby when they were around 16, I'd tell them to really think about it before they did it.

"I'd tell them, 'If you know like I know. Don't have one. It's not a toy you're talking about.

It's a child.'"

Two months before Patricia Lewis found out she was pregnant her mother complained that her attitude was changing, that her usually cheerful daughter was grumpy. Lewis' mother took her to the doctor.

"I was really scared when the doctor told me. I didn't know what my mother would think," says 18-year-old Lewis as she sat in her family's family room.

Lewis, who was 17 when she became pregnant, is two weeks overdue. And, she says, if the baby doesn't come soon, she plans to go to her senior prom.

"I think a lot about being scared. I'm frightened about the pain I will have to go through and I'm frightened about the responsibility of having a child."

Lewis says her pregnancy wasn't planned. She had dated the child's father for two years before going off birth control pills because she was haing medical problems. The next month she got pregnant. Now, a white straw bassinet stands cramped between a bed and dresser covered with reminders of the child soon to be born: tiny pastel socks, unopened baby bottles, a pink bottle of baby lotion.

Lewis' parents sleep in the room next door and the girl who dreamed of joining the Army says she will now stay with her parents until she can afford a place of her own. She doesn't want to get married, and she is no longer close to the child's father.

As scenes from "Dennis the Menace" and I Love Lucy" roll endlessly across a television screen, Lewis looks away as she describes her unhappiness about the pregnancy. But, she adds, she could not have an abortion or give the child up for adoption. She could not "kill a baby" or "give it up to strangers."

Instead, she talks about the child in phrases linked with extended families. She would rather give the child to her mother, than put it up for adoption she says. But when asked how her mother, who teaches full time, could care for a baby, the young girl in Lewis fights back tears and shrugs without answering.

"I never expected it to be like this and I guess maybe if I was older I would be happy. But if I was older I wouldn't be depending on my mother or family to help me.I would be doing it on my own, because I wanted to.

"Now I can't go out as much as I like and I miss my friends. I know when the baby is born if I want to do something, I will always have to think about having a babysitter.

"This could have been prevented, but now it's going to change my life. It makes me mad that I went off the pill.

"I guess I'm just another teen-ager who got pregnant and caught."