In this paper, factors that influence academic achievement and self-concept of Year 12 students in their final year of schooling at a Catholic co-educational school in Adelaide, Australia were investigated. The study hypothesised there were differences in attitudes and academic performance due to prior schooling experiences, sex, home experiences, peer influences and school policy and practices. The Self Description Survey (SDQ111) and aggregate results from four Year 12 assessment points were used in order to collect data. The data were examined using the specific statistical approach of effect size and correlational analysis. A focus on differences between those students who had previously had single sex schooling from those who had coeducational experiences over a five-year period found that difference existed with the scales of the SDQ111, particularly the Religion, Physical Appearance, Parent, Opposite Sex and General scales. Similarly there were clear sex differences noted on some of the 13 scales of the SDQ111 favouring the boys on the majority of these scales. One other factor of interest involved Parent Occupation. Evidence found that the parent's level of occupation was significantly related to academic achievement. (Contains 2 figures and 8 tables.)