Upper secondary school students with a strong academic self-concept are more likely to complete their studies and thus increase their well-being in the future. Previous research on the big-fish-little-pond-effect (BFLPE) has thoroughly established the negative contrast effect of average group academic achievement on students' academic self-concept. Many of these studies have been criticised for assuming the underlying social comparison without testing it. Here, survey and registered data from 1,047 Icelandic adolescents are used to explore the role of social comparison in the BFLPE. Models of hierarchical regression showed that students' ideas about their relative position within their group of students mediated the effect of group average achievement on academic self-concept. This strengthens the assumption of social comparison being the underlying factor of the BFLPE. No assimilation effect between the type of school and academic self-concept was found. Implications for educators, school authorities, and policymakers are discussed.