Through this research, we explored the relationship between student achievement and school environment. We compared mathematics scores from the PISA test in Ticino and Geneva. We discovered that in both cantons there is a significant but moderate school effect on student mathematics performance (around 7% of the variance is attributable to school affiliation). Based on our results, we rejected the hypothesis that the context effect was a social and/or academic compositional one. We identified other factors, related to the teaching staff (seniority) and to the organization (class size), which may partly explain the between-school variance. However, we did not identify a systematic effect, since none of the variables analyzed were found to be statistically significant in both cantons.