This study examined how psychosocial conditions at school are associated with prosocial behaviour, a key indicator of positive mental health. Participants were 3,652 Swedish Grade 9 students from the Health Behaviour in School-aged Children study. Structural equation modelling demonstrated that students who experience more manageable school demands and greater social support from teachers and classmates are more likely to display caring, sharing, and cooperative behaviours. However, those that feel acutely stressed, particularly girls, also reported greater prosocial behaviour. Teacher support played a greater role in girls’ prosocial behaviour and perceptions of school demands than boys’. The findings extend knowledge of the importance of psychosocial work conditions for adolescent health to positive mental health.