In a study of the effects of school structure on heterosexual behavior, self-report data on heterosexual behavior were collected from more than 2,000 sixth through tenth graders in 1978 when their district had a 6-3-3 school structure and in 1979 after the district had changed to a 6-2-2-2 structure. The district was located in a medium-size, rapidly growing, Midwest, middle to upper middle class suburban area with a high degree of marital stability. Students filled out a questionnaire on amount and type of dating, different types of sexual behavior, perceived peer pressure to date, and the relative age of members of the opposite sex that they had met or dated. In addition, the questionnaire asked students about their families, neighborhood, self-esteem, attitudes towards school, perceptions of the security offered by the school, and their engagement in such behavior as substance use, truancy, or vandalism. Results indicated that the absence of ninth graders from junior high school had almost no effects on eighth graders in terms of dating or sexual behavior and only minor effects on seventh graders in the direction of increased pressure to date for both sexes, increased amounts of dating for females and increased dating in couples for males. The presence of the older tenth graders in the new intermediate high school appeared to affect the dating and sexual behavior of ninth grade females in the direction of more dating and ninth grade males in the direction of more pressures to date, more dating in couples, and more sexual intercourse. (JMB)