The current landscape
There are approximately 163 grammar schools in England — selective state schools that admit pupils based on the 11+ examination. They are concentrated in particular local authority areas, including Kent, Buckinghamshire, Essex and Lincolnshire. No new grammar schools have been opened since 1998, though several existing ones have expanded.
The selection effect
Grammar school pupils do indeed achieve better academic outcomes than comparable pupils in non-selective comprehensives. The key question is why. Research suggests this is primarily a selection effect: grammar schools select more able, more advantaged pupils (whose parents are better placed to prepare them for the 11+), and those pupils would likely perform well in any school context.
What happens to those who fail
The evidence on pupils who narrowly fail the 11+ is concerning. Research by the Education Policy Institute found that selective areas have worse outcomes for pupils of below-average prior attainment than non-selective areas — the implied conclusion being that concentration of higher-ability pupils in grammars creates secondary schools with more challenging peer groups for the remaining pupils.
Social mobility and selective education
One of the most frequently cited justifications for grammar schools is social mobility — the idea that bright children from disadvantaged backgrounds get a pathway to elite universities. The evidence does not consistently support this: grammar schools significantly over-represent children from more affluent families (middle-class parents are better able to prepare their children for the 11+ and are more geographically mobile to live in catchment areas), and selective areas do not show better overall social mobility outcomes than non-selective areas.