Background In Colombia, the average age of first sexual intercourse is between 14 and 15, and it has one of the highest fertility rates among adolescents in Latin America, which reflects poor access to health services and a lack of knowledge about contraception. Some laws support comprehensive sex education as a learning process that families and schools should provide. The objective of this work was to assess the frequency of adequate knowledge and attitudes of acceptance towards contraception in students, their associated factors, sexual behaviours in sexually active students, and experiences with sex education. Methods Mixed methods study consisting of an analytical cross-sectional study and a qualitative hermeneutic phenomenological study. The former included students who completed a questionnaire with sociodemographic variables, surveys on knowledge and attitudes toward contraception, and questions about sexual behaviour. Focus groups were used in the qualitative study to evaluate students' and parents' sex education experiences. Findings 827 students were surveyed, of whom 52.3% had adequate knowledge about contraceptives and 80.1% had attitudes of acceptance. Talking with parents about sex education, receiving sex education, and being in the eleventh grade increased the likelihood of having adequate knowledge. Each year of age, being male, being a victim of forced displacement, and studying in a public school were associated with lower possibilities of attitudes of acceptance. Using condoms, compared to other contraceptives, was associated with less adequate knowledge and attitudes of acceptance (p < 0.05). Experiences with sex education, contraception, and sexual and reproductive rights were three emerging categories in the focus groups. Interpretation In a group of Colombian high school students, half of them had adequate knowledge, eight out of 10 had attitudes of acceptance about contraception, and both were associated with having received sex education. Nevertheless, different types of barriers to sexual education were identified at home and in schools. The results will allow the generation of educational policies that modify the educational model as well as new strategies by health professionals to raise awareness about responsible sexuality.
