Source: Selective evidence - Genes and backgrounds matter most to exam results - The type of school is less important
From this article: "Debate has raged for years over whether most selective schools do well because they provide a better education than state schools, or merely because they cream off the brightest and most privileged. According to research led by Robert Plomin and Emily Smith-Woolley, both of King’s College London, the educational benefits of selective schools largely disappear once the innate ability and socio-economic background of pupils at selective schools are taken into account. On average, the results of children at private or grammar schools were a full GCSE grade higher than those at state schools. That suggests attending a selective school gives children a boost. Without correcting for any other factors the researchers calculated the boost to be worth about 7.1% of the difference in GCSE results. But was this due to better teaching at these schools or an outcome of the selection procedure? To see, the team adjusted the grades based on the results of each child’s test scores, family circumstances and genes. Once they did this, the gap between the schools narrowed dramatically, with school type explaining just 0.5% of the difference in average GCSE grades. For any individual, genetics accounted for about 8% of the difference, modest in comparison with the many other factors involved, such as socio-economic backgrounds..."