The Living 4 Life study was a youth-led, school-based intervention to reduce obesity in New Zealand. The study design was quasi-experimental, with comparisons made by two cross-sectional samples within schools. Student data were collected at baseline (n = 1634) and at the end of the 3-year intervention (n = 1612). A random-effects mixed model was used to test for changes in primary outcomes (e.g. anthropometry and obesity-related behaviours) between intervention and comparison schools. There were no significant differences in changes in anthropometry or behaviours between intervention and comparison schools. The prevalence of obesity in intervention schools was 32% at baseline and 35% at follow-up and in comparison schools was 29% and 30%, respectively. Within-school improvements in obesity-related behaviours were observed in three intervention schools and one comparison school. One intervention school observed several negative changes in student behaviours. In conclusion, there were no significant improvements to anthropometry; this may reflect the intervention's lack of intensity, insufficient duration, or that by adolescence changes in anthropometry and related behaviours are difficult to achieve. School-based obesity prevention interventions that actively involve young people in the design of interventions may result in improvements in student behaviours, but require active support from leaders within their schools.