Studies investigating social comparison in the classroom have led to contradictory results concerning upward comparison effects. Research demonstrates that they can lead either to enhancing student's academic performance without influencing his/her selfconcept or to decreasing student's academic self-concept through Big- Fish-Little-Pond Effect (BFLPE). Our study tries to reconcile these results by further investigating social comparison effects relative to various frames of reference according to Stapel and Suls (2004) postulates. Effects of implicit or explicit social comparison on student's selfconcept and performance in Physical Education classes were thus considered simultaneously in the same study. Multilevel modelling analysis results demonstrate simultaneously positive and negative effects depending on the outcome and the explicitness or implicitness of the frame of reference considered. Counterbalancing the negative effects of the class-average level demonstrated in BFLPE studies, our results clearly provide support for assimilation effects of the small group comparison explicitly selected by the student on both selfconcept and performance.