Current cyberbullying literature lacks longitudinal studies clarifying its predictors and consequences. This 1-year longitudinal study investigated how social and emotional competencies develop according to Portuguese middle school students’ involvement in cyberbullying, and whether class size influences this relationship. There were 455 participants (Mage = 12.58; SD = 0.94; 46% girls), and data collection through self-reports took place in three different moments during 12 months. The results showed that students involved in cyberbullying in any role displayed negative trajectories during 1 year in self-control and social awareness, while victims and bully-victims displayed a more pronounced decrease in self-esteem and relationship skills during the same period. Additionally, girls displayed higher initial social awareness levels, while larger classes were associated with higher levels of self-control and responsible decision making. These results supported the importance of conducting longitudinal research and using a multilevel approach to address this topic.