Given that the Big-Fish-Little-Pond-Effect, the negative effect of school-average achievement on academic self-concept, is one of the most robust findings in educational psychology (Marsh, Seaton et al., 2007), this research extends the theoretical model, based on social comparison theory, to study relative year in school effects (e.g., being 1 school year ahead or behind same-age students) for math constructs in PISA2003 (276,165 15-year-old students from 10,274 schools across 41 countries). The effects on academic self-concept were negative for de facto acceleration (e.g., starting early or skipping grades) and positive for de facto retention (e.g., starting late or repeating grades). These negative effects of relative year in school were: (a) cross-culturally robust across all 41 PISA2003 countries (significantly negative in most, not significantly positive in any); (b) neither substantially explained nor moderated by a diverse range of control variables (e.g., gender, school starting age, repeating grades, home language, immigrant status, SES, achievement); (c) independent of the Big-Fish-Little-Pond-Effect. The negative effects of acceleration and the positive effects of retention are consistent with a priori predictions based on frame-of-reference research, one area of social comparison research, but apparently inconsistent with some popular beliefs in relation to policy/practice based on these variables.