Background: Emerging evidence suggests perceptions of being overweight account for many of the psychosocial consequences commonly associated with obesity, yet limited research has explored weight perception in associations between body mass index (BMI) and academic performance. The current study examined how BMI classification and weight perception relate to academic performance in a large cohort of youth.Methods: We used cross-sectional survey data from 61,866 grade 9-12 students attending the 122 Canadian schools that participated in Year 6 (2017/2018) of the COMPASS study. Mixed effect regression models were used to examine associations between studentsâ BMI classification and weight perceptions and their math and English/French course grades, stratified by sex.Results: For English/French grades, males and females with overweight or underweight perceptions were less likely to achieve higher grades than their peers with perceptions of being at âabout the right weightâ, controlling for BMI and covariates. For math grades, females with overweight perceptions, and all students with underweight perceptions, were less likely to achieve higher grades than their peers with âabout the right weightâ perceptions. All students with BMIs in the obesity range were less likely to report grades of 60% or higher than their peers with ânormal-weightâ BMIs. Overweight BMIs were predictive of lower achievement in females for English/French grades, and in males for math grades, relative to ânormal-weightâ BMIs. Results for students that did not respond to weight and weight perception items resembled those for obesity BMI and overweight/underweight perceptions, respectively.Conclusions: Overall, this study demonstrates that an obesity achievement gap remains when controlling for studentsâ perceptions of their weight, and that both underweight and overweight perceptions predict lower academic performance, regardless of BMI classification. Results suggest barriers to academic success exist among youth with larger body sizes, and those with perceptions of deviating from âabout the right weightâ.Plain Language Summary: An obesity achievement gap has been suggested as an early contributor to later socioeconomic disparities found by weight status. To date, limited research has examined how sociocultural weight norms contribute to potential links between weight status and academic performance. Emerging evidence suggests weight perceptionâindividualâs subjective appraisal of their body weightâaccounts for many of the psychosocial consequences commonly associated with obesity. We sought to determine if body weight and weight perceptions predicted grades in a large sample of Canadian secondary school students. It was hypothesized that perceptions of being at âabout the right weightâ would provide a protective effect. Overall, this study demonstrates that an obesity achievement gap remains when controlling for studentsâ perceptions of their weight, and that weight perceptionsâboth underweight and overweightâpredict academic performance, regardless of BMI classification. Results contribute to a body of research that encourages the consideration of both overweight and underweight perceptions and their potential impact on adolescent emotional and physical health. Further research is needed to determine the mechanisms underlying these relationships, in order to remove barriers to academic success among youth with larger body sizes, and those with perceptions of deviating from âabout the right weightâ.