The assumption that multi-grade learning enhances and sustains positive self-concept is widespread, although neither theory nor empirics have yet allowed for firm conclusions. This paper reports on a representative longitudinal study of multi-grade learning in grades 3 and 4 comparing the development of students' self-concept in reading in multi-grade and single-grade classes, also providing a differentiated analysis of the development at varying performance levels. The results show that self-concept is less stable in multi-grade classes. At the end of grade 3, students' self-concept in multi-grade classes is lower than in single-grade classes, although the average achievement level is higher. This effect is mainly ascribed to low-achieving children; however, all students’ self-concept recovers by the end of grade 4. Regarding students with a very low self-concept, they are found to a comparable extent in both multi-grade and single-grade classes, but the number of these students is low and the affected students change. The data indicates that pedagogical support is needed, especially in grade 3, to mitigate the effects of social comparison. Further analysis should include the quality of multi-grade teaching implemented in the classroom and its effects.