The study explores the influences of critical thinking and interests on students' performance at school. The tested students attended German grammar schools ("Gymnasien"). Separate regression analyses showed the expected moderate positive influences of critical thinking and interests on school performance. But analyzed simultaneously, applying a Structural Equation Model, only a direct effect of critical thinking on school performance was observed. Furthermore, critical thinking seems to be a moderator variable, mediating an indirect effect of interest on school performance. An additional analysis of the data showed that the influence of critical thinking could exclusively be observed in the subsample of students who had a family background without a migration history. In the subsample with migration history critical thinking and interests did not have an effect on school performance. Since the students with migration history did not differ in school performance from their fellow students without migration history, the result gives rise to the assumption that those students in German grammar schools may have chosen other ways of motivation and learning style to school performance. Further research was done to clarify why critical thinking turns out to be a mediator of school performance and if this function is observable regarding other variables and different age groups. Beyond that we suggest reconsidering some theoretical and empirical issues, especially reviewing the relationship between critical thinking skills and dispositions.