''The big trend in sex education is . . . thinking!'' A no-nonsense trainer of teachers in the field of sex education, Peggy Brick of Englewood, N.J., rolls her eyes with a laugh at the simplicity of her own statement. But for a half-dozen of her colleagues from around the United States who have joined her for an informal discussion of trends in public school sex education, Ms. Brick's words describe a welcome development.
''Today when we talk about sex education, we mean the broad picture: We're talking about self-esteem, responsibility, values, and, really, what we mean by femininity and masculinity,'' says Bill Stackhouse, director of the a program for parents at the Sex Information and Education Council of the US in New York City. ''We want people to think.''
Studies by the Urban Institute and researchers at Johns Hopkins University suggest that fewer than 10 percent of all students are involved in this sort of comprehensive sex education, or ''thinking'' course, even though about three-fourths of American students receive some form of sex education at school.
Yet, according to sex educators, the last few years have witnessed a slow shift away from purely ''informational'' courses to ones that encourage children , at steadily younger ages, to consider such brow-knitting topics as love, morality, decisionmaking, and aspirations.
As this transition has taken place, schools have done more to include their neighbors - religious leaders and public-service organizations such as the YMCA, Boys Clubs, and Scouts - to help develop programs that best reflect community concerns. Perhaps most important, more schools are encouraging parents to work with them in clarifying a topic that many mothers and fathers still find difficult to discuss with their children.
One result is that fewer school districts are experiencing the opposition to sex education that was prevalent in the 1970s. After years of concern that such classes teach children how to become sexually active, the general feeling today is that young people do need some training to deal with all the information about sex - often misleading, confusing, or exploitative - they receive from peers and the media.
According to Scott Thompson, executive director of the National Association of Secondary School Principals, courses covering reproduction and the obligations of marriage and family face little opposition now - although topics such as contraceptive use, homosexuality, and abortion remain controversial.
Parents express even greater support than the general public: In a 1981 Gallup poll, 79 percent of parents said they favored sex education; 66 percent of nonpar-ents were in favor. Says Ms. Brick, whose state mandates sex or family education for all grades (starting in kindergarten with the concept of home as the center of love): ''The media and child sexual abuse - both are factors that make parents very much in favor of family-life education. They want their children to know how to handle the messages they're constantly receiving.''
Just how well adolescents are dealing with those messages is open to debate. But the facts of adolescent sexual activity are not.
According to the Alan Guttmacher Institute, the research arm of Planned Parenthood, about half of all boys aged 15-17 and one-third of all girls in the same age group are sexually active. Studies done by the institute show that sexual activity among unmarried women aged 15-19 rose by two-thirds during the 1970s.
As a result, about 4 in every 10 become pregnant before they turn 20. For the great majority, these are unintended and unwanted pregnancies. In 1981, of some 1.1 million teen pregnancies about 434,000 were terminated by abortion. The birthrate among teen-agers has actually dropped by almost half since 1960; abortion, in part, but also the availability and use of contraceptives, are responsible for the decline.
Some sex-education teachers feel that the schools must distance themselves from the much-publicized problem of teen-age pregnancy. ''We should not see sex education in the classroom as preventing teen-age pregnancy,'' says Martha Roper , who has been teaching sex education for 10 years at the University City School District outside St. Louis. ''We did not cause that problem in the schools, and it's not going to be solved there.''
Notes Mr. Thompson, ''Attitudes about premarital sex are set by society at large. We can't expect schools to profoundly affect the posture of the general culture on these questions.''
Research generally shows that sex education has little influence on teen-agers' decisions to initiate sexual activity. But a number of studies indicate that it can reduce the number of pregnancies among sexually active adolescents. And there is some evidence that sex education can help the muddled adolescent solidify his conviction that sex is not right for him - especially when parents are involved in the discussion.
Yet in spite of clear indications from both parents and children that parents should be the primary providers of sex education, many parents remain reluctant. For this reason, some schools have begun bringing parents into their sex education curriculums. Konstance McCaffree, a sex education teacher at Council Rock High School in Newtown, Pa., says she spends 12 to 14 hours with the parents of students taking her classes.
''They have a lot of anxiety built up, much of it from the media,'' she says, describing the parents in a typical course. ''First I just get them talking with other parents. Then we cover general awareness of the teen-ager's world. And there's always homework, often involving the kids, so there'll be communication. That's the key.''
Ms. Roper follows a different approach. She holds an evening session to discuss with parents what their children will be learning in her class. ''We want the family to see us as promoting positive family relationships, as transmitting the moral values of the family and child.'' Ms. Roper outlines for parents four basic moral values that are promoted in her classes: love and caring for all humanity; respect for the family and religious values; responsibility for one's actions; and the idea that ''no one has the right to exploit another person.''
After realizing that information-giving courses are not enough, educators have placed greater emphasis on such topics as decisionmaking and aspirations. Increasingly they are finding that adolescents need to develop the ability to see themselves in real life - as opposed to the ''unreality'' of television, movies, and music lyrics - and to think seriously about the future and the consequences of their actions.
Joy Dryfoos, an associate professor at the Columbia University School of Public Health and noted researcher of teen-age pregnancy, says her work, especially among poor minority youth, has convinced her that a completed education and a broadening of economic options are keys to reducing teen-age pregnancies.
Today there are about 600,000 families with children five years old or younger headed by mothers under 26 years old. A 1975 study by the Alan Guttmacher Institute showed these families receiving about half of the year's $9 .4 billion in federal Aid to Families with Dependent Children. About two-thirds of them live below the US government's official poverty line.
Writing in Impact '84, the annual publication of the Institute for Family Research and Education at Syracuse University, Ms. Dryfoos says: ''If these children can be assisted to stay in school, learn to read and write, develop vocational goals . . . perhaps they will enjoy higher self-esteem, higher aspiration, and greater achievement such as advantaged children enjoy. . . .If these changes take place and a sense of future is engendered, the option of child-bearing will seem less acceptable.''