There are several factors that indicate the need for sexuality education during schooling. However, sexuality education programmes are very heterogeneous. The aim of this research is to quantitatively evaluate the effectiveness of school-based sexuality education programmes for adolescents. A meta-analysis was conducted on the scientific evidence of the effectiveness of school-based sexuality education programmes in different countries from 2000 to 2020. The PRISMA 2020 guidelines were used. The review included 47 studies. Effect size was calculated using Hedges’ g. Heterogeneity was analysed using Cochran’s Q test and the I2 statistic. Programmes increased attitudes towards sexuality and condoms, knowledge, condom self-efficacy, condom use intention and abstinence. Heterogeneity was high, so different moderator variables on the efficacy were analysed.