This paper adopts the technique of DiNardo, Fortin and Lemieux (1996) to decompose differences in the distribution of PISA test scores in Canada, and assesses the relative contribution of differences in the distribution of “class size” and time-in-term, other school factors and student background factors. Class size and time-in-term are both important school choice variables and we examine how provincial achievement differences would change if the Alberta distribution of class size and time-in-term prevailed in the other provinces. Results differ by province, and for provinces where mean achievement gaps would be lower, not all students would benefit.