This study was designed to examine academic self-concept as a potential mediator to better understand the relations between academic attitudes and academic achievement in early adolescence. A total of 1398 high school students from Alicante, Spain (47% female, M = 12.5 years), participated in the study. Multilevel mediation analyses with Monte Carlo confidence intervals were used to measure within-subject effects at the student level (L1) and between-subject effects at the class level (L2). Academic attitudes (attitudes towards teachers and attitudes towards school) and academic self-concept were measured with validated scales, whereas academic achievement was assessed using the end-of-term grades obtained by the students in nine subjects. The results show, first, significant effects of the academic attitude constructs on the mediational and dependent variables at both the within and between levels. Second, academic self-concept was an important mediator for all academic attitude constructs at both levels of analysis. These results highlight the importance of academic self-concept during early adolescence and suggest that academic attitudes are crucial for the future development of educational models.