Adolescence is a lifetime in which young people, adolescents, begin to separate from their parents and relate to peers, comparing them to each other, and following these comparisons they conclude themselves. They consider their sexual behavior to be personal choices, but they are still under the influence of parents, peers, societies, and cultures under whose norms it is determined. Adolescents thus remain confused and without real counsel about sex life and how to deal with it responsibly. Hypotheses set up for research are that medical school pupils have a lot of knowledge about sexually transmitted diseases and protect them so that girls later become sexually explicit and have more knowledge about sexually transmitted infections and that younger subjects have less understanding of sexually transmitted diseases. The research aimed to influence the influence of age and school environment on sexual behavior, and responsibility towards the health of the pupils of medical schools, economics schools, and gymnasiums. The same number of 1st and 4th-grade pupils participated in the study, of which 40% of male and 60% of female. The findings indicate that almost all students have information on sexually transmitted diseases and that most of the report provides in school and the media. The results obtained also show that there are no sex differences in the age of sexual intercourse. Older respondents (4th-grade students) have a more excellent knowledge of sexually transmitted diseases than younger ones (1st-grade students). Pupils of all three schools show no difference in the understanding of sexually transmitted diseases, only a statistically significant difference is found in the knowledge about spreading sexually transmitted infections by the oral route.