The aim of this study is to understand the behavior of school segregation by socioeconomic level in the UK. To do this, all data from the United Kingdom are analyzed in the PISA Assessment from 2000 to 2015 and the Gorard index, Dissimilarity index, and the Isolation index are estimated. The analysis has shown that socio-economic segregation between schools has declined somewhat in the UK from 2000 to 2015, although the clustering of the 25% poorest of students remained relatively static since 2006. England remains more highly segregated by poverty than Wales, Scotland and Northern Ireland. The segregation levels of the 10% poorest student has declined in state-maintained schools but shown a sharp increase in private schools. The level of isolation of disadvantaged students is less in state-maintained schools than private schools. The findings show that poverty segregation trends using PISA data match with segregation trends previously analysed using the national datasets using only state-maintained schools for England. This finding leads to research implications for a detailed analysis of national school segregation trends, including student data from private school.