This study investigated the social, emotional and mental wellbeing predictors of bullying victimisation and perpetration for boarding students following the transition from primary school to secondary boarding school. Longitudinal data on 76 male and 74 female boarding students in Grades 7, 8 and 9 was used from a larger longitudinal study of 3,462 students. A spike in frequency of bullying perpetration occurred immediately after the transition to secondary school and boarding school, with an increase in bullying perpetration continuing to occur throughout the following 2 years. Those boarding students reporting higher levels of depression, anxiety, and stress symptoms were significantly more at risk of frequent bullying victimisation at the beginning of Grade 8, while boarding students experiencing conduct problems and emotional difficulties were more likely to engage in frequent bullying perpetration. Peer support and feeling safe were found to be factors most likely to decrease frequent bullying victimisation. The sustained nature of bullying behaviour highlights the importance of addressing attitudes towards bullying both prior to and immediately after the commencement of boarding school. It is recommended that boarding schools adopt a targeted and systematic whole school approach to bullying prevention, while promoting positive interactions and relationships with peers.