Interventions that identify and develop character strengths have been shown to benefit well-being, academic engagement and achievement. Strengths research within positive psychology has focused primarily on individual outcomes with less attention on group or relationships effects. This study (N = 193) examined the effect of a classroom-based strengths intervention on class cohesion and friction, relatedness, engagement, well-being and strengths use. A six-session programme was trialled with 9–12-year-old students. Students learned to recognise strengths and practised strengths-related goal setting. At three-months post-test, multi-level modelling indicated that intervention group participants scored significantly higher on class cohesion and relatedness need satisfaction, and lower on class friction than the non-randomised control group. Programme participants also reported higher levels of positive affect, classroom engagement, autonomy need satisfaction and strengths use. School-based strengths interventions may influence individual perceptions of class climate, engagement and student relatedness in addition to individual well-being.