Research on the big-fish-little-pond effect demonstrates that class-average achievement negatively affects students' academic self-concept via social comparison processes. The neighborhood-effects literature reports positive effects of advantageous socioeconomic neighborhood conditions on students' academic development via collective socialization mechanisms. To investigate how socioeconomic neighborhood conditions affect academic self-concept, we separately and simultaneously analyzed classroom- and neighborhood-level composition effects on students' academic self-concept, using two samples drawn from two grade levels (N[subscript Grade5] = 3,906, N[subscript Grade9] = 3,277). Analyses of the neighborhood level only indicate that socioeconomic neighborhood conditions negatively predict general, math, and German self-concept in Grade 5. In Grade 9, this holds only for math self-concept. In simultaneous analyses including classrooms and neighborhoods, socioeconomically advantageous neighborhood conditions negatively predicted general and math self-concept in Grade 5.