This dissertation focuses on the impact of class size on students’ academic performance in public secondary schools in Geita Town Council. The specific objectives were (1) to establish the relationship between class size and students’ performance, (2) to determine the challenges facing class size, and (3) to establish the intervention measures to overcome the challenges of related to class size. The study involved 110 participants. These included town secondary education officers, heads of schools, teachers and students. Simple random, stratified, and purposive sampling was employed to select the participants. The study used mixed research approach. Descriptive survey design was used to provide wide insight about class size and performance. Data was collected through questionnaire, semi-structured interviews, and observation. Quantitative data collected was coded and analysed using SPSS version 16. Qualitative data was analyzed thematically by screening and transcribing the themes to match the research question. The findings revealed that the challenges facing large class size are: difficulty in monitoring students’ attendance and checking students exercises, classroom management, noise level, teaching and learning materials, absenteeism, difficulty to control students’ behavior and attitude, difficulty making evaluation and assessment, and low participation of students in the class. It was concluded that teachers and students prefer small classes for better performance rather than large classes. The findings show that in order to have good students’ performance in large classes; the following should be done: the government should build more learning facilities such as classrooms, laboratories, libraries and provide more text books. In addition, the government should recruit more qualified teachers to reduce student-teacher ratio, and they should provide more learning and teaching materials. The study recommends that the ministry of education should provide more funds for learning facilities in order to enroll students according to the recommended ratio which is 40 students per class.