This study focuses on socially unequal school choice of public and private elementary schools. This is relevant because private schools in Germany must be open to children of all social classes (Article 7(4), Basic Law), but social differences in attendance at private schools have been observed. The aim is to provide explanations for these socially unequal school attendance rates. The data comes from a survey of parents of future first-graders in a large German city (Erfurt). Theoretically, this paper draws on Kristen's (2005) three-stage model of school choice, which was applied to the choice of public and private schools. The first stage shows that parents with lower education do not perceive private schools as alternatives. At the second stage, it is found that parents with lower education report above average that they cannot afford private schools. There are also differences in utility expectations by educational level with regard to the quality of private schools and in school choice motives. However, these can only explain the social differences in private school application at stage 2 to a small extent. The third stage shows that academic parents are more likely to apply successfully to a private school than non-academic parents.