This study analyses the effects which repeating a class has on ninth grade students’ development of mathematical competency. The following research questions were addressed: How many students repeat grades in the different types of schools? How do students who repeat a grade differ from those who do not in their performance and background characteristics? How much extra mathematics do students repeating a grade learn in one school year? What are the differences between various types of school? Can students with poor mathematics grades in particular profit from repeating a grade? The sample is a sub-sample of the PISA-I-Plus study and comprises N = 360 ninth grade students. The total sample of PISA-I-Plus is representative for all ninth/tenth grade students from the different school types in Germany. The data survey was carried out in the ninth grade and then repeated after the students had repeated a year. The results document differences in the amount of grade repeat quotas between types of school. Furthermore, compared to students not repeating, those repeating a grade had lower mathematics (d = 1.02) and german (d = 1.14) grades, a lower level of mathematical literacy (d = 0.51), and lower test results with regard to basic cognitive abilities (d = 0.32). In terms of the development of mathematical literacy, the students repeating a grade could improve by an average of 23 points (d = 0.27) on the PISA mathematics scale. However, the results identify 38 percent of students repeating a grade who do not make any significant improvement in mathematics or even get worse. A differentiation according to school types shows that students repeating a grade in integrated comprehensive secondary schools and in schools with several educational levels in particular do not, on average, show any noteworthy improvement in their mathematical literacy. The analysis of the school grades received in mathematics shows that students whose mathematics grades are unsatisfactory do not benefit more from repeating a grade than students whose mathematics performance has been rated as being “satisfactory” or better. The article concludes with a discussion of the possible consequences of changing the way in which repetitions of grades are dealt with.