Research on students’ understandings of their academic performance often faces limits with respect to sample diversity, statistical power, breadth of participant information, and ability to continuously track the development of participants. Government registry data do not face such limitations. We validate a brief measure of academic self-perceptions contained within the Danish Well-Being Survey, a self-report measure administered annually to all Danish public-school students (grades 4 through 9) and linked with rich registry data regarding these students, their families, schools, and communities. We then perform exceptionally well-powered analyses of the influence of academic self-perceptions on the pursuit of further academically-intensive education (N = 35,227) and of the development of academic self-perceptions during late childhood and adolescence (N = 284,024).