Academic self-concept plays a central role in successful learning and is substantially shaped by social comparisons. Research on the so-called Big-Fish-Little-Pond Effect (BFLPE) has yielded a highly robust and generalizable pattern of negative effects of higher class/school average achievement on students' self-concept when controlling for individual achievement. However, most BFLPE studies have not provided information about the causes behind or the mechanisms underlying the proposed effects. To address this, we used a fully immersive virtual reality (IVR) classroom to experimentally test the extent to which students recognized performance-related classroom behavior as implicit social comparison information and how these perceptions explained differences in students' self-concepts. Participants (N = 381 sixth-grade students) experienced an authentic yet standardized IVR teaching situation with virtual classmates who exhibited different performance levels (operationalized as 20% vs. 35% vs. 65% vs. 80% of classmates raising their hands). Hand-raising behavior had a significant positive effect on students' perceptions of the class' performance level (d[subscript 20% vs. 65%] = 0.60; d[subscript 20% vs. 80%] = 1.24). In line with the BFLPE, results showed a negative effect of higher performing classmates on students' situational self-concept (d[subscript 20% vs. 80%] = 0.30). Students' perceptions of the class' performance level fully explained the effect of classmates' hand-raising behavior on students' situational self-concept. The study's experimental approach provided new insights into the emergence of social comparison effects in the classroom, highlighting the major role of students' perceptions of their classmates' performance-related behavior, and moreover demonstrated the general potential of using IVR in classroom research.