Consistently with a priori predictions, school retention (repeating a year in school) had largely positive effects for a diverse range of 10 outcomes (e.g., math self-concept, self-efficacy, anxiety, relations with teachers, parents and peers, school grades, and standardized achievement test scores). The design, based on a large, representative sample of German students (N = 1,325, M age = 11.75 years at Year 5) measured each year during the first 5 years of secondary school, was particularly strong. It featured 4 independent retention groups (different groups of students, each repeating 1 of the 4 first years of secondary school; total N = 103), with multiple posttest waves to evaluate short- and long-term effects, controlling for covariates (gender, age, socioeconomic status, primary school grades, IQ) and 1 or more sets of 10 outcomes collected prior to retention. Tests of developmental invariance demonstrated that the effects of retention (controlling for covariates and preretention outcomes) were highly consistent across this potentially volatile early to middle adolescent period; largely positive effects in the first year following retention were maintained in subsequent school years following retention. Particularly considering that these results are contrary to at least some of the accepted wisdom about school retention, the findings have important implications for educational researchers, policymakers, and parents.