Being schooled with other high-achieving peers has a detrimental influence on students’ self-perceptions: School-average and class-average achievement have a negative effect on academic self-concept and career aspirations—the big-fish-little-pond effect. Individual achievement, on the other hand, predicts academic self-concept and career aspirations positively. Research from Western and developed countries implies that the negative contextual effect on career aspirations is mediated by academic self-concept. Using data from the Program for International Student Assessment (PISA) 2006 (a total of 398,750 15-year-old students from 57 countries), we test the generalizability of this mediation model in science using a general multilevel structural equation modeling framework. Individual achievement was positively related to academic self-concept (52 countries) and career aspirations (42 countries). The positive effect on career aspirations was mediated by self-concept in 54 countries. The negative effects of school-average achievement on self-concept (50 countries) and career aspirations (31 countries) also generalized well. After controlling for self-concept at both the individual and the school level, there were significant indirect contextual effects in 34 countries— evidence for mediation of the contextual effect of school-average achievement on career intentions by academic self-concept.