Montgomery County school teacher remembers the day her 12-year-old son came home from school and told his mother he and his friends knew all about the best way to keep a girl from getting pregnant.

"Oh?" she remembers asking.

"Sure," her son told her, "Saran Wrap."

"That," says Doris Moon, education director for Montgomery County Planned Parenthood, "is where most kids still get their sex education. From their peers. And that is precisely the kind of misinformation they get."

Moon has been working with teen-agers and sexuality issues for almost a decade and she says sadly that little has changed. "If we could get to parents, we could help them be comfortable talking about it, could tell them where to go for more facts.

"It isn't the parents' fault," she says, "because 80 percent of parents want their children to have some sexuality training, but it is our society. On the one hand we're terribly permissive about things like 'Dynasty' or the daytime soaps, but in our homes we're still really mid-Victorian."

"Even so," she says, "many parents feel it is important to keep the sexuality education of their children in their own hands. However, either because they don't have the information, or are embarrassed or they were raised without any special education, it just doesn't happen.

"Many of them just do the 'big talk,' which is usually something like, to a girl, 'Well, uh, you're going to menstruate and then you, uh, have to be careful.'

"As I talk to young people, as I face them day in and day out, I ask them if they can talk easily to their parents. I'm lucky if I get 10 percent who say they can."

The Center for Population Options, a national nonprofit educational organization in the District devoted to the problem of unwanted teen pregnancies, says that although "most parents believe they have a major responsibility in communicating sexuality information to their children, many feel uncomfortable in discussing sexual topics and would like help in doing so."

CPO found that in a survey of 1,400 parents of children aged 3 to 11, fewer than 15 percent of the mothers and 8 percent of the fathers had ever talked to their children about premarital sex and intercourse. In a 1978 study, 80 percent of mothers with daughters aged 11 to 14 had talked about menstruation, but only 4 percent had explained in any detail the relationship between menstruation and pregnancy.

So the myths abound. In addition to Saran wrap, Moon and Jan Adams, an obstetrical/gynecological nurse-practitioner at the Cygma Health Center in Kensington have picked up such widely disseminated misinformation as:Douching with Coca-Cola can prevent pregnancy.You can't get pregnant the first time. Jumping up and down after sex prevents pregnancy.You can't get pregnant if you have intercourse standing up.You can't get pregnant seven days before and seven days after your period.

"I believed that one once myself," admits Adams, who has specialized in working with parents and teens -- helping the former tell the latter about sex without embarrassment and without recourse to the "big talk."

Adams will be running a four-session seminar for parents on Wednesday evenings Nov. 13 and 20 and Dec. 4 and 11 titled "Sexuality: Parents as Primary Educators." The seminars, $10 per person per session, are sponsored by and held at the Cygma Health Center in Kensington.

One technique that Adams has used successfully with parents is to give them a "homework" assignment to remember their first date, the first time they held hands, the first kiss, "not necessarily how old they were the first time they had intercourse at all, just those early moments of awakening sexual feelings."

"If you can get parents to recall their own tentative, scared feelings, the more they will be able to identify with their children. The kids aren't out there necessarily saying we want to have intercourse, they're just having new and scary feelings. And the more we can promote positive feelings about the developmental experiences, then, hopefully, the more responsible everyone will be in the behaviors they choose."

According to the Center for Population Options, although sexual activity has increased among teen-agers, with 11.6 million teen-agers between 13 and 19 years of age having had sexual intercourse, only about half of sexually active teen women used any kind of contraceptive the first time. Sexual activity among unmarried teen-age women increased by about 67 percent during the 1970s, says CPO, and "if current trends continue, 40 percent of today's 14-year-old girls will bcome pregnant at least once before the age of 20."

One fifth of all premarital pregnancies among teens occur within the first month after com- mencing sexual intercourse, and half occur in the first six months, a CPO fact sheet says.

Teen-agers who have intercourse before they are 16 years old are nearly twice as likely to get pregnant in the first six months than are those who begin sexual activity at 18 or 19.

There are more than 1 million teen-age pregnancies every year, of which about three-quarters were unintended.

Jan Adams believes that the lack of open ended communication of all kinds -- not just about sexuality -- is at least part of the problem, and she has enlisted a Rockville-based mime troup, Teens Educating About Challenges and Hope (TEACH), to participate in the seminar sessions.

For example, the mimes will set up a situation in which a teen-ager is late coming home. Then the actors will "freeze," and two members of the audience will be drawn on stage to play out the drama. "Parents," says a Cygma brochure, "are a child's earliest models of sexuality and authority. Because most parents did not learn about sexuality from their parents, they have no role models to help them approach their own children." The seminars are designed to help parents provide teen-agers with the "basic knowledge necessary to survive in this 'sexual society.' "More Information

To register for the seminars, call Cygma, 949-3900. For information on teen-agers and sexual considerations, write Center for Population Options, 2031 Florida Ave. NW, Washington, D.C. 20009.