Private supplementary tutoring is a widespread phenomenon. However, evidence that private tutoring has positive effects on academic achievement or about the specific conditions of successful private tutoring is rare. Adapting Carroll's (1963) model for school learning to private tutoring, we expected to find positive effects of tutoring duration, tutoring intensity, and students' motivation to attend private tutoring. In a sample of eighth-grade students in German secondary schools (N = 8510, 18.6% currently being tutored), we conducted regression analyses with multiple covariates and did not find a positive main effect of private tutoring attendance in any of the school subjects examined. Moreover, within the subsamples of tutored students, we were not able to identify positive effects of tutoring duration, tutoring intensity, tutoring content (such as a focus on homework completion, test preparation, or study behavior), or students' motivation to attend private tutoring. Given these disillusioning findings, we primarily derive suggestions for future research.