The German Medical Society for Health Promotion (ÄGGF) has developed aschool-based teaching unit for students in grade8 and higher that aims at primary and secondary prevention of unplanned pregnancies. The accompanying evaluation study analyzes the effects of the teaching unit on the students' knowledge and their experience with self-efficacy concerning unplanned pregnancies. Atwo-armed cluster-randomized controlled study was conducted using aten-page anonymous questionnaire with aclosed answer format before and after the intervention. In the first survey, 1855students participated; 1523students participated in the second. Participants in the intervention group showed agreater increase in knowledge over time. Compared to the control group, they showed an increased intention to use the pill and condoms as well as amarked increase in self-efficacy in the case of an unplanned pregnancy. Furthermore, they significantly corrected their perception of the age at which same-aged boys start having sexual intercourse. All effects remained stable when covariates were controlled. The teaching units were highly accepted: more than 90% of the participants stated that they would recommend them to other students. Aschool-based medical teaching unit about the prevention of unplanned teenage pregnancies was able to contribute to the improvement of students' knowledge and competency. The intervention itself was highly accepted within the target group.