The study purpose was to investigate determinants of academic performance in Kenya day secondary schools in Gilgil sub-county. The study objectives were, peer pressure influence on academic performance, to assess teacher effectiveness affects students’ performance, to find out how school resources influence academic performance, family background influence academic performance the study used the survey. The target population for this study consisted of 300 students where only 80 students participated in the study, 20 school principals and 180 class teachers. The sample was selected randomly, the study also incorporated use of questionnaires which were structured containing closed ended and open-ended questions. The study was guided by the following specific objectives: To determine how peer pressure affects the academic performance of students in day secondary schools, to investigate how teacher effectiveness has an effect on students` academic performance, to find out how school resources influence the academic performance of a student and to assess how different family backgrounds have an effect on student` academic performance. Quantitative data was analyzed using descriptive statistical technique. Qualitative data was categorized and reported in emergent themes hence the findings will be presented by use of frequency distribution tables that give record of time a score or a response occurs. The findings revealed that effective teaching practices, including thorough lesson preparation and regular feedback, were crucial for enhancing students’ understanding and Performance. Approachability and support from teachers significantly contributed to students’ academic success. Adequate learning materials and well-maintained school facilities were essential for creating a conducive learning environment. Limited access to technological resources like computers and the internet hampered academic performance.