Class size remains an important dimension of the learning environment, particularly in public secondary schools where increasing enrolment can place substantial pressure on available teachers, classrooms, and instructional resources. This study examined the effects of class size on students’ academic performance among selected public secondary schools in Nigeria. Specifically, it compared academic achievement across small, medium, and large classes and considered teacher attention, student participation, classroom noise, and student satisfaction as relevant classroom-process variables. An ex-post facto comparative research design was adopted. The study comprised 274 students drawn from selected public secondary schools across Lagos, Oyo, Rivers, Enugu, the Federal Capital Territory (FCT), and Kano. Classes were classified as small (≤35 students), medium (36–55 students), and large (≥56 students). The groups comprised 92, 91, and 91 students, respectively. Academic performance was assessed using students’ average academic scores and subject-specific scores in English Language and Mathematics, while grade and student satisfaction provided supplementary outcome measures. Analysis of the supplied dataset revealed a pronounced gradient in academic performance across class-size categories. Students in smaller classes consistently recorded higher academic scores, while students in medium-sized classes demonstrated intermediate performance and those in large classes recorded the lowest scores. The classroom-process data similarly indicated that smaller classes were characterised by greater teacher attention, higher student participation, lower classroom noise, and higher student satisfaction. Conversely, large classes were associated with low teacher attention and participation, greater classroom noise, and lower satisfaction. The pattern was also evident in English Language and Mathematics scores. These findings indicate a strong association between class size and academic performance in the sampled schools and suggest that classroom conditions such as teacher attention, participation, and noise may constitute important pathways through which class size relates to learning outcomes. Given the ex-post facto design, however, the findings should be interpreted as evidence of association rather than definitive causal effects. The study concludes that effective management of student–teacher ratios and classroom enrolment should remain an important component of policies aimed at improving teaching and learning conditions in Nigerian public secondary schools.