State secondary schools in England are not doing enough to stretch the most able students, Ofsted says.
A report from the education watchdog says gifted pupils are not keeping up with their clever counterparts in independent and grammar schools.
Children are not "realising their early promise", says Sean Harford, Ofsted's national director of schools.
But heads' leader Russell Hobby said it was "neither sensible nor accurate" to accuse schools of failing.
Inspectors looked at how well pupils who had been high-flyers at primary school made the transition into secondary school.