By assessing differentiated upper secondary education with homogeneous student backgrounds, previous studies indicate that a high concentration of students from families of higher socioeconomic status (SES) and a climate of educational expectations for higher educational attainment in schools result in a "hot house" effect that facilitates students' shadow education participation. Building on the literature, this study investigates whether the effect exists in an egalitarian system as a possible hidden mechanism of inequality in learning opportunities due to de facto socioeconomic disparities among residential places. Using a nationally representative sample of ninth graders in Japan, this study identifies the hot house effect on shadow education participation in the compulsory education system, which is regarded as egalitarian. Specifically, family SES relates to parents' educational expectations, neighborhood SES appears to shape varying neighborhood-level expectations, and these factors lead to differentiation in shadow education participation, countering the intention of the egalitarian education system.