The number of schools in England judged to be failing has increased by more than 18% over the past year.
Inspectors can make spot checks on schools
But more are being visited under the new shorter inspection system, and the rate of failure fell from 2.7% to 2.2%.
By the end of the last summer term, 246 schools were in "special measures" - the most serious category - compared to 208 at the end of summer term 2006.
But data from schools inspectors Ofsted showed a slight fall in the numbers needing "significant improvement".
By late August, 306 had been given what is called "notice to improve", compared to 312 at the same point last year.
Ofsted said there were 246 schools in special measures - failing to provide a satisfactory education - at the end of the summer term 2007, compared with 208 at the end of the summer term 2006.
However, as more schools were inspected in 2006-07 than in 2005-06, the proportion of schools inspected that went into special measures was lower: 2.2% in 2006-07 compared with 2.7% in 2005-06.
The total, 246 schools failed that year, was 1.1% of all schools in England whereas 208 the year before was 0.9%.
School inspections have got tougher in recent years. Schools which might have been judged "satisfactory" before, are now seen as not good enough. Ofsted says it has raised the bar.
Schools now have shorter inspections, lasting about two days and carried out at short notice. Schools which give cause for concern are given extra checks.
In total, by late August this year, 552 schools in England were either in special measures or had been given notice to make significant improvements.
This is in the context of there being 22,000 maintained schools in England.
Among those in special measures, 181 were primary schools, 47 were secondary schools, nine were special schools and nine were units for pupils with behavioural problems (PRPs).
Those given notice to improve were 203 primary schools, 86 secondaries, seven special schools and 10 PRPs.
Schools Minister Andrew Adonis said: "Parents expect us to take an uncompromising approach to schools that are underperforming."
Things had improved significantly over the past decade, more than halving the number of failings schools since 1998 from 515 to 246.
"But we must press on and reduce even further the numbers of weak and failing schools.
"Our reforms to turn around failing schools demand radical action from the school and the local authority.
"A school in special measures has to turn around in 12 months otherwise the school could face closure."
The head of the Association of School and College leaders, John Dunford, said Ofsted openly admitted that it had made it tougher for schools to do well in inspections.
"Therefore it is misleading to try to make like-for-like comparisons using 2006 and 2007 data.
"Despite the fact that Ofsted has raised the bar on inspection gradings, the number of secondary schools deemed to be 'failing' has fallen [by 13%].
"This represents a great achievement by the people leading these schools, especially those in challenging circumstances."
Shadow schools minister Nick Gibb said the new short-notice inspections were catching more failing schools overall.
"It is clear that the new-style Ofsted inspections are picking up failure which in the past may have been left unnoticed," he said.
"This is a welcome development as it will lead to higher standards in the long