This paper adopts the technique of [DiNardo, Fortin and Lemieux (1996). "Labour market institutions and the distribution of wages 1973-1992: A semiparametric approach." "Econometrica, 64"(5), 1001-1044.] to decompose differences in the distribution of PISA reading 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. (Contains 4 tables, 4 figures, and 9 footnotes.)