Objective: To determine the impact of a peer-led, school-based programme (Students As LifeStyle Activists; SALSA) on energy balance-related behaviours (EBRBs) in Grade 8 students, and the cost of implementing the programme. Design: Pre-post test Setting: High schools in New South Wales, Australia. Methods: Grade 10 students (15- to 16-year-olds) were trained by university students to deliver four lessons on healthy eating and physical activity to Grade 8 students (13- to 14-year-olds). Students completed an online questionnaire pre-post lessons on EBRBs and intentions to change EBRBs over the next month. Items included fruit, vegetable and sugar-sweetened beverage (SSB) intake, frequency of eating breakfast, participation in moderate-to-vigorous physical activity (MVPA) and school-day recreational screen-time. Pre--post changes were analysed using generalised estimating equations, adjusted for clustering. Standard methods were used to estimate implementation costs. Results: In total, 2,056 Grade 8 students from 23 high schools participated. Significant changes included 5.2% increase in eating >2 fruit serves/day (p < 0.001), 2.5% increase in eating >4 vegetable serves/day (p < 0.05), a 4.3% increase in drinking <1 cup/day of SSB (p < 0.001) but limiting screen-time <2 hours/day decreased by 4.4% (p < 0.001). There were significant improvements in students' intentions to change EBRBs over the next month, with the exception of MVPA. The average actual cost of implementing the SALSA programme in 2014-2015 was AU$1,388 (US$958) per school and AU$9.97 (US$6.88) per student. Conclusion: The SALSA peer education programme had a positive impact on most of the dietary EBRBs examined. The cost evaluation showed that it is a relatively low-cost programme to implement.