In Israel, fertility rates are very high in relation to developed countries. As a result, Israeli families are relatively large, and class size is high. These two variables are known in the literature as factors limiting scholastic achievement. Using PISA 2000 and PISA 2006 data we show that the relative size of the young population in Israel explains most of the gap between the scores of Israeli students and the international average. Number of siblings and mean class size mediate the effect of the size of the young population. While studies on achievement usually focus on the education system itself, this study highlights the importance of the demographic context to the understanding of educational outcomes of Israeli students.