Comprehensive sexuality education (CSE) is increasingly recognised as a key component of school-based health promotion, aiming to equip young people with knowledge, attitudes, and skills to navigate relationships, sexuality, and well-being in a respectful and informed way. Despite the growing support for CSE in policy and research, less attention has been paid to the fidelity and quality of its implementation in real-life classroom settings. Process evaluation offers valuable insights into how such programs are delivered, including contextual and relational factors that shape their impact. This paper presents the results of a case study focused on the process evaluation of a school-based program grounded in the principles of CSE, implemented in four schools (ten classrooms) in the Czech Republic. The program targets students in lower secondary education (aged 11–15) and aims to support the development of knowledge, skills, attitudes, and values related to sexuality, relationships, and emotional well-being.

The purpose of the study was to document the implementation process using the established process evaluation model. Specifically, we describe: (1) the school and class context in which the program was implemented, including the process of school recruitment; (2) the program model, including its theoretical foundations, stated objectives, and delivery methods; and (3) the implementation in three key areas—lecturers’ training, program fidelity (defined as adherence to the curriculum protocol and the quality of implementation), and student engagement.

To examine these areas, we employed a case study design, allowing for an in-depth exploration of the implementation process in real-life school settings. Data collection methods were tailored to the specific focus of each evaluation area. Lecturers’ training was examined through training documentation and post-training reflective feedback. Program fidelity was assessed by analysing lesson plans, lecturer self-reports, and observation notes. Student engagement was explored through structured classroom observations conducted by an external evaluator using a pre-defined observation tool. All collected data were analysed using qualitative content analysis, enabling the identification of key themes, recurring patterns, and context-specific features.

The findings highlighted key contextual and implementation-related factors influencing program delivery. In the area of lecturers’ training, reflective feedback pointed to the importance of sufficient preparation and peer support, particularly when delivering emotionally or socially sensitive content. Regarding program fidelity, differences in curriculum coverage were observed across classrooms and were influenced primarily by group dynamics and the delivery style of lecturer pairs, rather than by time constraints. Student engagement was generally high, with students actively participating in interactive activities, quickly adopting new terminology, and showing a noticeable shift toward more respectful and inclusive attitudes, especially during value-oriented exercises such as myth-busting. The overall classroom atmosphere emerged as a key factor shaping the program's impact, offering valuable insights for future program adaptation and lecturer development.

Keywords: Comprehensive Sexuality Education, Process Evaluation, School-Based Prevention, Student Engagement, Programme Implementation.