This study investigated the duration in school and academic performance. This was a comparative study of the 3 and 4-year education duration in Ghana. The purpose of the study was to determine which of the two durations of SHS (3-year or 4-year) had positive influence on students' academic performance, in the three selected schools. The quantitative method was used for the study. A population of 5836 students and 149 teachers who taught both the 3 and 4-year SHS students and 3 headmasters were drawn from three schools in Central Region via census, purposive sampling techniques respectively. The documentary checklist and structured questionnaires were used to collect the data. The raw WASSCE scores of 5836 students and the questionnaire data were analysed quantitatively with SPSS. One research question and two hypotheses were formulated to guide the study. Mean and standard deviations, frequencies, independent and paired sample t-test, and oneway were used to analyse the data. The major findings of the study were that the duration of schooling has a positive effect on students' academic performance and the 4-year cohorts of students performed better than the 3-year cohorts. The MOE and GES should consider the 4-year duration as a better suited duration for SHS education in Ghana and should therefore be implemented again.