The UK government is planning to increase the number of pupils attending state-funded selective grammar schools, claiming that this will assist overall standards, reduce the poverty attainment gap and so aid social mobility. Using the full 2015 cohort of pupils in England, this article shows how the pupils attending grammar schools are stratified in terms of chronic poverty, ethnicity, language, special educational needs and even precise age within their year group. This kind of clustering of relative advantage is potentially dangerous for society. The article derives measures of chronic poverty and local socio-economic status segregation between schools, and uses these to show that the results from grammar schools are no better than expected, once these differences are accounted for. There is no evidence base for a policy of increasing selection, and so there are implications for early selection policies worldwide. The UK government should consider phasing the existing selective schools out.