She entered 10th GRADE with the rest of us and about two months later she disappeared abruptly and at first no one thought too much about it. The word was that she'd gone away to private school for the year. Next fall she returned to school. In the 10th grade. We saw her in the hallways, and wondered why she was still in the 10th grade, but no one asked. Later on we heard why she had been away. We didn't have sex educations in the homes in those days, much less in the schools, but we'd all read the Scarlet Letter. We knew why she had gone away.

That was back in the late '50s, when 90,000 illegitimate children were born to teen-agers each year, when the majority of those babies were bundled up and hurried off to adoptive or foster parents, when the mothers were branded and the whole episode was shrouded in shame.

Important changes have occurred since then and there is a move afoot to see that public policy reflects those changes. The number of unmarried teen-aged mothers has more than doubled - they gave birth to 235,300 children in 1976 - and 93 percent of them kept their children, according to a study by Johns Hopkins University professors Melvin Zelnik and John F. Kantner. They found that white girls, who traditionally gave up their babies, are now keeping them, just as black teen-agers have done historically. In 1971, 72 percent of the white babies remained with their mothers. In 1976, 87 percent remained.

Unwed teen-aged mothers in the '60s, pressured by tradition, tended to relinquish their babies, according to Jean Connor of the Florence Crittenton Home in Washington. "In that decade, girls weren't very brave about the rearing of a child alone. Of the girls we had in residence - 350 to 400 a year - all but about 7 percent relinquished their children. Then, as the '70s progressed, each year we've found a larger number keeping their children."

The teen-aged mother of the '70s is a different kind of young woman, one who is unwilling to undergo the wrenching process of pregnancy, birth and abandonment, but who is willing to undertake the awesome task of raising a child. Many of these young mothers continue to live with their families - a child who has her own child - and it is within these families that researchers are finding an untapped reservoir of strength, help and knowledge.

Four such families agreed to be interviewed on film by family researchers from the Philadelphia Child Guidance Clinic. The film was shown in Washington recently at a conference on teen-age pregnancy sponsored by the Family Impact Seminar - a small, high-powered think tank that is examining how public policy in such areas as flexitime and teen-age pregnancy affects families. Their message surprised a conference of experts: Out-of-wedlock babies don't ruin girls and destroy their families anymore.

Quite the contrary. The babies mended tense relationships between grandmothers and daughters, soothed conflicts between the adolescent mother and her siblings, and helped her mature.

"If anything, it made my life better," said one 18-year-old mother. "It seemed before my mother and I didn't get along. Now we seem to get along."

The grandmother described her daughter as a former discipline problem, a girl who did poorly in school, refused to do household chores, stayed out late at night. When she became pregnant, the family did not ostracize her. It rallied around her - surprising the daughter who assumed her parents would want her to abort. "I think she matured more with the pregnancy," said the grandmother.

There are four other children in that family.