Gaps in GCSE attainment have long been the concern of policy makers, academics, and social commentators, largely due to the importance of these exams for setting children on their future academic and career pathways. In the past a wide range of factors relating to the pupils, their families, and their schools have been found to account for differences in GCSE attainment. In this paper we examine the role of pupils' beliefs in their own academic ability (academic self-concept). Using Next Steps data, we examine whether pupils with higher academic self-concept do better or worse in their GCSEs than pupils with lower academic self-concept. Results show that on average, controlling for other characteristics, having high academic self-concept increases GCSE scores by four grades. When we compare academic self-concept to measured achievement we find that both high and low attainers have higher probabilities of achieving five A*-C GCSEs and higher GCSE point scores on average if they have high academic self-concept than similarly able students who have lower academic self-concept.