The principle of schools collaborating to improve is one that has seen growing interest in recent years, and there is emerging evidence that in particular collaboration between high and lower performing schools can be an effective school improvement method. However, this evidence relates primarily to secondary schools, and little research has been conducted on the factors that could make collaboration more or less effective. In this study we specifically looked at partnerships between low and high performing primary schools, in which high performing schools acted as supporters to low performing partner schools. A mixed methods approach was used. A quasi-experimental quantitative study was conducted to ascertain the relationship between partnership and pupil attainment using data from the National Pupil Database. This was followed up by case studies of nine partnerships. Findings showed that there was a positive relationship between partnership and pupil attainment at Key Stage 2, and that successful partnerships were characterised by intensive interventions focused on teaching and learning and leadership.