By Sean Coughlan
BBC News education reporter
Ed Balls says the academy scheme has helped tackle low achievement
The number of underperforming secondary schools has fallen sharply in England - as the government pushes ahead with plans to improve the weakest schools.
There are now only 270 schools below the threshold of 30% of pupils achieving five good GCSEs, including English and maths.
This is a 40% year-on-year reduction in schools in this struggling category.
Schools Secretary Ed Balls said "to get this number to zero would once have seemed an impossibility".
In 1997, there were about 1,600 schools which had not met this level of achievement. In the past 12 years, it has fallen from about one in two secondary schools to one in 12.
"There were some schools which had a view of the world that low expectations were accepted," said Mr Balls.
He said the government would be "uncompromising" in ensuring that all schools rose at least to this minimum level.
"This is a floor target - not the summit of our ambition for schools," he said.
There will now be drives to tackle underachievement in specific local authorities - Kent, Leeds and Suffolk.
Mr Balls said this summer's GCSE results were evidence of the success of the National Challenge scheme which set a minimum level of exam results.
This threatened schools which failed to meet this benchmark by 2011 with closure or being replaced by a school with a new identity, such as an academy.
Mr Balls said the academy programme had been a "genuine revolution in how secondary education is delivered for those areas and pupils that need it most".
However, underperforming academies which had been running for at least three years would face the same compulsion for improvement - including the threat of closure - as other schools, he said.
At present, there are 40 academies below the threshold for results.
Mr Balls said his advisers would be working with education officials in Kent - where about one in five secondary schools is below the government's threshold.
He said that he did not accept the argument that non-selective schools would not succeed in selective areas such as Kent.
The plan to systematically target the lowest-performing schools and to replace those that did not improve was first put forward by Prime Minister Gordon Brown - who promised to "eradicate failure".
It angered teachers' unions which argued that this would stigmatise schools working in the toughest circumstances.
The general secretary of the National Union of Teachers, Christine Blower, criticised the national challenge initiative.
"Naming and shaming schools is an unacceptable measure which does nothing for the morale of teachers, pupils and the local community," she said.
Nansi Ellis, head of education policy at the Association of Teachers and Lecturers, said the national challenge scheme "appears to have given schools additional, targeted support to improve results, and it is vital that schools continue to get the support they need to sustain improvements".
"The government should end its continuing obsession with academies as the answer to all and any education problem."
Shadow schools secretary Nick Gibb said: "There are still far too many schools where fewer than a third of children reach the basic standard of five good GCSEs including English and maths, and it is the poorest areas that are worst affected."