This study examined the impact of classroom environment on senior secondary school students in selected public secondary schools in South-East Nigeria, with particular emphasis on the mediating role of student academic engagement in the relationship between classroom environment and academic performance. The study conceptualized classroom environment in terms of physical environment, psychosocial climate, and availability of instructional resources, while academic engagement comprised behavioral, emotional, and cognitive engagement. A quantitative cross-sectional correlational design was adopted. The study involved 850 Senior Secondary Two (SS2) students drawn from 85 public secondary schools across Abia, Anambra, Ebonyi, Enugu, and Imo States. Data on classroom environment and student engagement were measured using structured scales adapted from established classroom-environment and engagement instruments, while academic performance was assessed using students' mock examination scores. Attendance rate was incorporated as an additional indicator of participation in schooling. Data were analyzed using descriptive statistics, Pearson product-moment correlation, multiple regression, and mediation analysis with bootstrap estimation. The hypothesized model specified classroom environment as the independent variable, student academic engagement as the mediator, and academic performance as the principal outcome variable, while gender, class size, and teacher qualification were treated as control variables. The analytical framework indicated positive relationships among classroom environment, academic engagement, and academic performance. Improvements in classroom physical conditions, supportive teacher-student and peer relationships, and access to appropriate instructional resources were associated with stronger behavioral, emotional, and cognitive engagement, which in turn was associated with improved academic achievement. The study underscores the importance of viewing classroom environment as a multidimensional educational condition capable of influencing students' participation, motivation, learning experiences, and academic outcomes.