I had mentioned the issue only in passing: that the Reagan administration plans to send registered letters to parents when their daughters under 18 are given birth control devices from federally funded family planning clinics. In the high school class I was teaching, hands shot up. A discussion began. The kids wanted to talk. I, for one, wanted to listen.

Among the 25 students in this public high school class, many were young women eligible to be served by any of the nation's 5,000 family planning clinics. They were eligible, too, to speak out, except that as the courts, lawyers, public health workers and Reagan officials debate the proposal due to become law Feb. 25, the kids would be the last to be heard.

Not today though, not in my class. I asked: do you think the Reagan administration has your best interests in mind? Will families be helped or hurt by the proposed rule? After we talked, I asked the students to put their thoughts on paper.

This is a sampling: The rule "disgusts me. Families will not be helped, but they will be made--lots and lots of little families running around. . . . Telling women of any age that they cannot have abortions is one thing--a topic that can be reasonably debated. But telling a woman under the age of 18 that she cannot have sex without her parents' consent--which in my mind is what this regulation is saying --is disgusting, and criminal and completely nonsensical. Not to mention the fact that it tells of lot of teen-age girls who want to be mature and take responsibility for their actions that they do not have the right."

Another: "I think this regulation is absolutely absurd. If a girl wanted her parents to know, she would tell them. I think this will result in more pregnancies and more hassles for teen-age women."

A third: "Many parents don't know that their children are sexually active, and the children are afraid to tell their parents. Many girls will be afraid of what their parents might think and do and therefore not buy birth control. If girls are afraid to buy birth control, then more babies (unwanted babies) will be born."

And this: "What bothers me is that the same people who make birth control hard are the ones who are against abortion. It seems to me that if you are against abortion a simple solution would be to make birth-control products as easy to buy as aspirin. Then no one would have an unwanted pregnancy. The new rule seems to be self-defeating."

None of the students believed that the Reagan administration is remotely close to understanding the complexities of the sexual options available to adolescents. From the discussion, as well as the written papers, the kids looked on the rule as still another hypocritical adult finger waving a no-no in their faces.

The simplism behind the rule is the unfounded notion that availability of birth control devices to teen-agers leads to rampant sex. The idea that contraceptives cause promiscuity is on a level with the view, once dear to Ronald Reagan, that trees cause pollution. Adolescent chastity is a sound ideal, despite the routine scoffing at sexual purity. The ideal in no way conflicts with the proven usefulness of family planning clinics.

In ruling to hell with confidentiality, the administration says that its registered letters are ways to ensure that parents involve themselves in the sex and health lives of their children. No serious research is available to back up this naivet,e. It is mere blind-eye guessing that registered letters can create either responsible parents or responsible sex.

The shelves bend under the weight of other research. One study reports that when it comes to sex ed, fathers are tongue-tied: 25 percent of boys and only 2 percent of girls have been talked with by Dad about sex and contraception. Maternal guidance isn't much better. A California study of mothers of seventh-grade girls found that "the data from this study seem to support findings from prior studies which have consistently indicated that children get little sex education in the home."

A letter from the government--federal snitching--isn't likely to get parents and children talking. If anything, it will start them shouting.