This study examined how family, peer and school factors are related to different trajectories of adolescent alcohol use at key developmental periods.Latent class growth analysis was used to identify trajectories based on five waves of data (from grade 6, age 12 to grade 11, age 17), with predictors at grades 5, 7 and 9 included as covariates.Adolescents completed surveys during school hours.A total of 808 students in Victoria, Australia.Alcohol use trajectories were based on self-reports of 30-day frequency of alcohol use. Predictors included sibling alcohol use, attachment to parents, parental supervision, parental attitudes favourable to adolescent alcohol use, peer alcohol use and school commitment.A total of 8.2% showed steep escalation in alcohol use. Relative to non-users, steep escalators were predicted by age-specific effects for low school commitment at grade 7 (P = 0.031) and parental attitudes at grade 5 (P = 0.003), and age-generalized effects for sibling alcohol use (Ps = 0.001, 0.012, 0.033 at grades 5, 7 and 9, respectively) and peer alcohol use (Ps = 0.041, < 0.001, < 0.001 at grades 5, 7 and 9, respectively). Poor parental supervision was associated with steep escalators at grade 9 (P < 0.001) but not the other grades. Attachment to parents was unrelated to alcohol trajectories.Parental disapproval of alcohol use before transition to high school, low school commitment at transition to high school, and sibling and peer alcohol use during adolescence are associated with a higher risk of steep escalations in alcohol use.