The major aim of this study was to investigate the effect of insecurity of school environment on the academic performance of secondary school students in Imo state, Nigeria. A total of 1000 made up of 500 each of male and female students responded to a self-structured validated questionnaire designed for the study. Two research questions and two hypotheses were formulated to guide the study. Means and standard deviations were calculated to answer the research questions and independent samples t-tests were used for testing the hypotheses. Major findings revealed that insecurity of school environment significantly affects the academic performance of secondary school students while students' gangsterism, smoking of Indian hemp, abusing other hard drugs, cult and related violent activities were some of the factors that constituted insecurity of the school environment which eventually cause boys to leave school and join trading while leading girls to drop out and settle for marriage. Based on the findings, it was recommended that owners of schools and other stakeholders in education should take bold steps to fence and protect school environments from intruders to ensure safety of the students.