This paper examines the link between the clustering of long-term disadvantaged students within schools, and the attainment gap at age 11 between these disadvantaged students and the rest. The data comes from the National Pupil Database for England from 2006 to 2019. The analysis focuses on students who would go on to be officially recognised as living in relative poverty for all 11 years from when they arrived at primary school up until age 16. This subset of disadvantaged students is a stable proportion of around 4.4% of each age cohort. They would have attracted Pupil Premium funding for their schools in any year, if it had been available, and despite any legal or economic changes over time. Comparing the educational outcomes for this group in comparison to their peers therefore provides a fair test of the impact of the Pupil Premium funding policy on the clustering of the poorest children, and their attainment relative to their peers. The segregation of long-term disadvantaged students between primary schools declined nationally after the policy was introduced. This happened in all economic regions, especially in areas with higher proportions of such pupils, for all ethnic groups, and for students with a special educational need or disability. This drop is strongly correlated over time and place with a corresponding decline in the attainment gap at age 11. It provides potentially strong evidence of the benefit of a targeted additional funding scheme like the Pupil Premium, with implications for how such funds are best used worldwide.