The study investigated the development of sources of self-efficacy in self-regulation in 9-12 year old (N = 317, primary school) students during one school year. We used latent growth curve modeling to determine (1) how the different sources of self-efficacy for self-regulation (i.e., mastery experience, vicarious experience, social persuasion, physiological and emotional arousal) change over one school year, and (2) how students' gender, special educational needs, and behavioral and emotional strengths relate to these changes. These results indicated that, on average, mastery experiences decreased and physiological and emotional arousal increased, whereas vicarious experience and social persuasion remained stable. Girls had higher initial levels of physiological and emotional arousal than boys, and experienced a steeper decline in mastery experiences compared to boys. Students with special educational needs had lower initial levels of mastery experience and higher physiological and emotional arousal than students without special educational needs. In addition, students with higher individual strengths presented higher initial levels of mastery and vicarious experiences, and lower physiological and emotional arousal. The results contribute to current research and practice by indicating that sources of self-efficacy for self-regulation change over time and are thus malleable in the school context.