Writing skills are important for educational achievement, employment and social and civic participation. This quasi-experimental study aimed to improve essay writing skills with an evidence-based intervention in two schools of mainstream P6 (grade 5) students (N = 44) in a largely rural local authority in Scotland. The intervention included peer revision of each other's texts in a programme of writing strategy instruction and self-regulation. The control students (N = 46) followed an on-going parallel intervention. The experimental programme led to large positive effect sizes for writing quality. This was the first study in Scotland to investigate writing strategy instruction and self-regulation as part of an evidence-based intervention.