Background and Context: Vygotsky's sociocultural theory emphasises the importance of language, mediation, and the transfer of skills and knowledge from the social into the cognitive plane. This perspective has influenced the development of PRIMM (Predict, Run, Investigate, Modify, Make), a structured approach to teaching programming. Objective: The objective of the study was to find out if using PRIMM to teach programming had an impact on learner attainment in secondary school, and the extent to which it was valuable for teachers. Method: We evaluated the use of PRIMM in 13 schools with 493 students aged 11-14 alongside a control group, using a mixed-methods approach. Teachers delivered programming lessons using the PRIMM approach for 8-12 weeks. Data were collected via a combination of a baseline test, a post-test to compare control and experimental groups, and teacher interviews. Findings: Learners performed better in the post test than the control group. Teachers reported several benefits of the PRIMM approach, including that PRIMM helped them to teach effectively in mixed-ability classes, enabling all learners to make progress. Implications: PRIMM makes a contribution to programming education research by building on previous work in effective pedagogy for teachers, and encouraging the use of language and dialogue to facilitate understanding.