This study sought to determine whether the academic self-concepts of children come from teacher appraisal or their own cognitive abilities. Longitudinal data from the Millennium Cohort Study were used to answer this question by testing the internal/external frame of reference (I/E) model for English children aged 5, 7, and 11 years and of ethnic minority, disadvantaged, and advantaged backgrounds. The I/E model predicts that high verbal achievement leads to a high verbal self-concept but a low mathematical self-concept, and high mathematics achievement leads to a high mathematical self-concept but a low verbal self-concept. The results of structural equation modelling revealed that the I/E model was generally supported, but teacher assessment had greater effects on self-concept than did cognitive ability. Teacher assessment and cognitive ability measured in children at the age of 7 years predicted child self-concepts at 11 years old. Teacher assessment was less influenced by the type of measurement used but more influenced by the cultural backgrounds of the children than cognitive ability. The implications of these results are discussed herein.