The relationship between social-emotional experiences -- number of friends, competitiveness, and self-worth -- and changes in bullying behaviour was investigated among students in Grades 5 to 7. This two-wave study included 235 students for the first data collection and 237 students for the second data collection, with a final sample of 227 students who had participated at both time points. Following Zumbo, results of multiple regression analyses, using residualized difference scores, indicated that changes in the school variables (number of friends, competitiveness, and self-worth) were associated with changes in reported bullying behaviour from the middle to the end of the school year. In particular, children's beliefs about their self-worth were important in predicting changes in bullying behaviour, with increased self-worth being associated with a decrease in reported bullying behaviour.