This study investigated the relationship between class size and students’ participation on the academic performance of senior secondary school students in Nigeria. The population for this study comprised all the Senior Secondary school students in Mainland Local Government Area, Lagos State. The study adopted descriptive survey design and random sampling technique was used in arriving at the selection of two hundred and seventy (270) students and thirty (30) teachers among the population of senior secondary school students in Mainland Local Government Area in Lagos State, which formed the sample of this study. Specifically, students and teachers were randomly selected from six Senior Secondary schools in Mainland Local Government in Lagos State. Forty five (45) Senior Secondary school students and five (5) teachers were randomly sampled from each school. Two self- developed, structured and validated questionnaires (students’ questionnaire and teachers’ questionnaire) of 15 items for teachers’ questionnaire and 18 items for students’ questionnaire were used to collect information from the students and teachers after its validity and reliability were determined. The data collected were analyzed using mean, standard deviation, simple percentage while Pearson Product Moment Correlation Coefficient statistical tool was used for hypothesis testing at 0.05 levels of significance. The study revealed that: there is no significant relationship between class size and academic performance. There is significant difference between the students’ participation and their academics performance in secondary schools. There is no significance relationship between students’ perception of class size and the academic performance. There is no significance relationship between teachers’ perception of class size and the students’ academic performance.