Omega-3 fatty acids are essential for brain function. We recruited 409 children aged 3-13 years (M=8.27, SD=2.17) for a randomised controlled trial supplementing with placebo or fish oil capsules (providing 750mg docosahexaenoic plus eicosapentaenoic acids, and 60mg gamma linolenic acid/school day) for 20 school weeks (Phase 1) followed by one-way crossover to fish oil (Phase 2). Children undertook assessments of reading, spelling and non-verbal cognitive development (Draw-A-Person) at baseline, 20 and 40 weeks. Teachers completed Conners Behaviour Rating Scales (CBRS). The treatment group showed improvements in Draw-A-Person compared with the placebo during Phase 1 (p=0.029), with strongest effects in Indigenous 7-12 year olds (p=0.008). The placebo group showed significant within-group improvements after switching to treatment (p<0.001). There was no treatment effect for reading or spelling, and CBRS data were unable to be analysed. These findings may be understood in the context that sustained school attendance and nutrition interact to produce school-related achievement.