In order to better understand the negative causal relationships between childhood overweight and obesity and school achievement, we implement a qualitative analysis based on 29 adolescents interviewed in Mexico City. Our findings suggest the following. First, excess weight affects schooling outcomes through two interdependent channels: social stigmatization and loss of productivity. Second, three main mechanisms seem to be involved in the relationship. While the loss of productivity alone represents one of such mechanism, the social stigmatization of obesity passes through two mechanisms: (i) a direct social mechanism, including material aggressions and exclusion by peers, and (ii) an indirect social mechanism, referring to the psycho-sociological troubles that stigmatized people may express. Third, the risk of weight discrimination depends on different individual factors: age, gender, fat distribution and body-mass category.