In this thesis, I examine how families, schools, and education systems shape inequalities in children’s school achievements. I show that in the United States, low-SES children fall behind their peers from better-off families over the course of primary and middle school. This is true even for those low-SES students who perform at similar levels than their peers from better-off families at the time of school entry. Why are these children left behind? Does this happen because they are raised in families that are more disadvantaged than those of their peers from better-off families, or because they attend schools of lower quality than those of their higher-SES peers? To separate the effects of families and schools, I compare learning that takes place during the summer holidays to learning that takes place during the school year. During the summer holidays, schools are closed and learning mainly reflects family influences. During the school year, both schools and families influence learning. Thus, the influences of these two institutions on learning can be disentangled by comparing summer learning and school-year learning. In addition, I examine parents’ educational behavior, finding evidence of their compensatory behavior when their children perform poorly. To determine whether the effects of schooling vary among countries, I compare these effects in the United States and Finland. In the United States, schools are segregated and of varying quality, whereas in Finland, there are relatively small differences between schools in terms of their student intakes and quality. To avoid overstating the effects of schools, I compare summer learning and school-year learning in both countries. I find that in Finland, the lower level of socioeconomic inequality between families helps to explain the higher level of education opportunity. Moreover, Finnish schools are better able to compensate for a disadvantageous family environment than are schools in the United States. To determine whether the socioeconomic inclusiveness of an education system benefits disadvantaged students but harms high-performing students or those from better-off families, I analyze how changes in the level of socioeconomic inclusiveness of the education system affects high- and low-SES students. Based on my findings, I conclude that whereas socioeconomically inclusive education systems benefit disadvantaged students, high-SES students perform well everywhere.