A variety of factors are associated with students' achievement in secondary school physics, including cohort size - the total number of students studying Physics in the school - and socio- economic status. Earlier studies also showed boys achieving better in Physics, while more recent research has shown better results for girls. Statistical analysis of the results of 25,682 Year 12 physics students from 99 schools in the Perth metropolitan area explored three factors related to performance in the physics course. Results were also analysed in terms of the schooling sectors - state, Catholic, high fee non-Catholic (grammar school) and low fee non-Catholic (smaller Christian and other independent schools) - from which students were drawn. Cohort size and socio-economic status were found to be strongly correlated with physics success, while sex was not found to have a significant correlation with achievement.