Academic achievement of students especially at the secondary school level is not only a pointer to the effectiveness or otherwise of schools but a major determinant of the future of youths in particular and the nation in general. The medium through which the attainment of individuals and the nation’s educational goals can be achieved is learning. Learning outcomes have become a phenomenon of interest to all and this account for the reason why scholars have been working hard to unravel factors that militate against good academic performance. The purpose of this study was to investigate the class size and school climate as correlates of secondary school students’ scholastic achievement in Itesiwaju Local Government Area of Oyo State, Nigeria. Descriptive research design was used in the study. Six hundred respondents were selected from selected secondary schools in Itesiwaju Local Government Area of Oyo State, Nigeria. The respondents were measured with relevant standardized scale (instruments) which include class size scale, school climate scale and student performance scale with strong reliability coefficient and the data obtained was analyzed using Pearson Product Moment Correlation (PPMC) statistical analysis of the Statistical Package for the Social Science (SPSS). Two research hypotheses were raised and answered in the study. The result showed that there was significant relationship between the class size and secondary school students’ scholastic achievement (r= .883; P<0.05) and there was significant relationship between the school climate and secondary school students’ scholastic achievement (r= .755; P<0.05). On the strength of these findings, it was stressed and advocated the need for the public and private schools to develop moderate and appropriate class size for the student in the school and that they should make the school climate and environment conducive for the students’ scholastic achievement and overall development in the school. This will