Chemistry education in Rwandan secondary schools faces challenges, especially after the adoption of a competence-based curriculum that prioritizes student-centered learning. Traditional teacher-centered approaches often result in poor student performance. This study investigates the impact of structured group discussions on chemistry achievement in senior high schools in Kigali, addressing gaps in student engagement. Using a mixed-methods quasi-experimental design, 50 students were divided into an experimental group (taught with group discussions) and a control group (taught with traditional methods). Data were collected via a Chemistry Achievement Test, student questionnaires, and interviews with 6 science teachers and 2 administrators. Quantitative data were mainly analysed using independent statistical t-tests, while qualitative data were thematically analysed. The findings showed that students in the experimental group significantly outperformed the control group, suggesting group discussions effectively enhance chemistry learning. Furthermore, the results from the effect size calculations suggest that group discussions had a meaningful positive impact on student performance in chemistry, indicating the effectiveness of this teaching strategy. The study also revealed that smaller group sizes (four to six students) promote more productive discussions, and that group discussions foster peer learning and deeper subject understanding. These results offer new insights into the benefits of group discussion in chemistry education, which have been underexplored in previous studies. This study suggests that integrating group discussions can improve both academic performance and student participation in chemistry classrooms.