Adolescents are at risk of sleep deficit, which has serious consequences for their daytime functioning. However, school-based interventions to improve sleep have shown limited success. This might be due to the content of the programmes (e.g., not targeting central factors such as daytime stress and technology use) or because changes have not been captured due to a lack of long-term follow-ups. Hence, the aim of this study was to evaluate the long-term effects of a school-based sleep education curriculum including time-management training. The study used a quasi-experimental design. Participants were 3,622 adolescents (mean age 13.7, 48% girls); 286 were in the intervention group and 3,336 were followed as a natural control group. Data were collected before the intervention and at a 1-year follow-up. We divided participants into three groups according to baseline sleep duration (calculated from self-reported bed- and wake times, minus sleep onset latency): insufficient (<7 hr), borderline (7-8 hr) and adequate (>8 hr). Adolescents in the intervention group were ~2 times less likely to report insufficient sleep at follow-up as compared to controls. Sleep knowledge improved significantly in the intervention group but there were no changes in emotional sleep hygiene (e.g., bedtime worry) and perceived stress. Surprisingly, technology use increased and behavioural sleep hygiene worsened in the intervention group. Although the mechanisms of change need further investigation, the results of this study point to potential long-term benefits of school-based sleep programmes.