The measurement of children's academic achievements and the explanation of differences between social classes should not be dismissed by sociologists of education. Although inequality is a major theme of the field, the sociology of education has lacked empirical evidence on the structure of disparities in academic achievements. This is partly due to the difficulties involved in collecting sufficient data on academic achievement through schools. In and after 2002, studies were begun on the relationship between academic achievement and social class in Japan. At the time, schools were being heavily criticized within the context of the debate over falling children's academic achievements. Some significant surveys were administered at that time, though they were small in number. However, they left some important issue to be solved. The first is that analyses of the determinants of academic achievement are inadequate for clarifying what factors will diminish class differences in achievement. The second concerns the reliability and validity of the variables collected. In particular, variables on the economic conditions of households are lacking. Finally, the surveys were conducted only in large cities. This paper examines the factors that affect children's academic achievements, and the extent of the effect of such factors, through an analysis of the data of the Japan Education Longitudinal Study 2003 (JELS2003). JELS2003 was conducted in two areas: one a middle-sized city within the capital metropolitan areas, and the other a small local city. It also contains variables about the economic conditions of households. The major findings of the paper are as follows. 1. In the small local city, the differences of academic achievement between social classes were relatively small. 2. In the middle-sized city within the metropolitan area, children's academic achievements were affected by the level of monthly educational expenses, level of educational expectations of the child, and income level of the family. Inequalities in children's academic achievements in our society should be grasped in the context of the substitution of "parentocracy" for meritocracy.