Schools are a common setting for adolescents to receive health education, but implementation of these programs with high levels of completeness and fidelity is not self-evident. Programs that are only partially implemented (completeness) or not implemented as instructed (fidelity) are unlikely to be effective. Therefore, it is important to identify which determinants affect completeness and fidelity of program implementation. As part of the launch of Long Live Love+ (LLL+), an online school-based sexuality education program for adolescents aged 15-17, we performed a process evaluation among teachers and students to measure the levels of completeness and fidelity, identify factors influencing teachers' implementation, and to evaluate the students' response. Sixteen Biology teachers from nine secondary schools throughout the Netherlands who implemented LLL+ were interviewed and 60 students participated in 13 focus group discussions. Results showed that teachers' completeness ranged between 22-100% (M = 75%). Fidelity was high, but many teachers added elements. Teachers and students enjoyed LLL+, particularly the diversity in the exercises and its interactive character. The most important factors that influenced implementation were time and organizational constraints, lack of awareness on the impact of completeness and fidelity, and student response. These factors should be taken into account when developing school-based prevention programs.