England's state secondary schools are still failing their brightest pupils, inspectors have warned.
Tens of thousands of children are not achieving their potential, let down by a culture of low expectations, unchallenging work and disruptive classmates, according to a highly-critical Ofsted report.
It found that there has been "disappointing" progress in dealing with these problems since the watchdog first raised the issue two years ago.
Ofsted's study concluded that clever children in non-selective secondary schools are still not being pushed to achieve the very highest results compared to youngsters attending grammar or private schools.
In 2014, almost two-thirds (61%) of pupils who gained a Level 5 in English at the end of primary school - one above the expected standard - did not get an A or A* in the subject at GCSE. Around 32,000 (23%) of these pupils achieved a C or lower in GCSE English.
The picture was similar in maths, with almost a quarter of pupils who achieved a Level 5 at age 11 failing to gain at least a B in the subject at GCSE.
More than two-thirds (68%) - about 62,556 clever pupils - did not get an A* or A in both subjects at GCSE, the report adds.
"Our most able students in non-selective schools are still not being challenged to achieve the highest levels of scholarship," it said.
Inspectors found "too much complacency" in schools, with leaders happy that their bright pupils were making the expected progress and a lack of aspiration for these youngsters.
In nearly half of the schools visited, headteachers were not prioritising the needs of able pupils early enough while youngsters were often left simply "treading water" when they started secondary school.
"Not only did many of the most able students spoken to during this survey say they felt unchallenged by the teaching they received, they often said that low-level disruptive behaviour from other pupils affected their learning," Ofsted noted.
This was particularly the case in mixed-ability classes.
The damning report also found that clever students, especially those from poorer backgrounds, were not routinely getting decent advice and guidance to help them prepare for their future, with inspectors saying it was "worrying" that in four of the schools visited bright teenagers were not being encouraged to apply to top universities.
"Ultimately, because too many secondary schools are failing to get those students who are most able and disadvantaged off to a good start, fewer of these students are achieving top grades at GCSE, then studying A-levels and going to the most prestigious universities," Ofsted said.
"Only 5% of disadvantaged students who completed Key Stage 5 (A-levels) in 2012 went on to the top universities."
There are some schools where not a single able pupil gets the grades usually preferred by prestigious institutions, the report suggested.
It also found that bright boys still lag behind their clever female classmates and that able youngsters do worse when they attend a school where there is a small proportion of high-achieving pupils.
Sean Harford, Ofsted's national director of schools, said: "This report has focused particularly on those identified as the most able.
"While inspectors found pockets of excellence, too many of these children are not being challenged sufficiently - and thousands of highly-performing primary pupils are not realising their early promise when they move to secondary school."
He added: "It is especially disappointing to find that, almost two years on from our first report, the same problems remain.
"I hope school leaders see this report as a call to action - and raise the bar higher for their most able pupils, so that they can reach their full potential."
Ofsted's report was based on visits to 40 non-selective secondaries and 10 primaries, plus surveys and interviews.
Sir Peter Lampl, chairman of the Sutton Trust, said: "Today's report demonstrates an urgent need for more dedicated provision for the highly able in state schools. Ofsted is right to describe the situation as 'especially disappointing'. Too many of our brightest students are being let down."
Russell Hobby, general secretary of the National Association of Head Teachers, said: "The secondary sector has been subject to massive structural change over the past few years. It's neither sensible nor accurate to accuse secondary schools of failure. The system itself is getting in the way of success.
"We face an extended period of volatility. The cause of this volatility is the sheer scale and speed of changes to the examination system - changes to both the scoring of the exams themselves, and to the way these scores are used to judge the performance of schools.
"On top of this, we face a profound change to calculating secondary school performance with the introduction of the Progress 8 measure. The baseline for secondary school progress - the KS2 SATs - is also changing dramatically over the coming years.
"Not all of these changes are bad. The concern is that the scale and pace of them will make it very hard indeed to know what will happen and how the changes will interact."
A Department for Education spokesman said: "We know that the best schools do stretch their pupils. They are the ones with a no-excuses culture that inspires every student to do their best.
"Our plan for education is designed to shine a bright light on schools which are coasting, or letting the best and brightest fall by the wayside.
"That is why we are replacing the discredited system which rewarded schools where the largest numbers of pupils scraped a C grade at GCSE.
"Instead we are moving to a new system which encourages high-achievers to get the highest grades possible while also recognising schools which push those who find exams harder."