By Sean Coughlan
BBC News education reporter
Christine Gilbert rejected criticisms of the education watchdog
School improvement in England is being held back by a "stubborn core of inadequate teaching", says the annual report of education watchdog Ofsted.
But the chief inspector Christine Gilbert hit back at local authority leaders who had accused Ofsted of "feeding fears" over child safety.
Ms Gilbert said criticism of children's services was justified if it "saves just one child from abuse".
The report also raised concerns over maths and English in primary schools.
Chris Keates, leader of the NASUWT teachers' union, attacked the focus on negative aspects of the report.
"This regular 'talking down' of teachers and state schools is not only totally unfair, it is grossly inaccurate," she said.
Ofsted's annual report, based on 40,000 inspections, covers the range of the watchdog's remit - not just schools and colleges, but also childcare and children's services.
Ahead of the publication, the Local Government Association strongly attacked Ofsted's approach to inspecting children's services - accusing the watchdog of protecting its own reputation rather than young people.
OFSTED ANNUAL REPORT
Schools: 19% outstanding (15% last year)
50% good (49%)
28% satisfactory (32%)
4% inadequate (5%)
Childcare: 65% outstanding or good (56%)
FE colleges: 63% outstanding or good (72%)
Children's homes: 64% outstanding or good (64%)
Serious case reviews: 28% good; 72% adequate or inadequate
Ms Gilbert rejected such accusations - and said "Ofsted must not pull its punches".
"There is a small but increased minority of councils that are performing poorly, principally because they are not ensuring that children are as safe as possible," she said.
"Ofsted has become more demanding of local authorities - and I make no apology for this - because if our work saves just one child from abuse, torture or death, then this is justified."
The assessments of children's services in local authorities found 32% were either adequate or inadequate - with 61% good and 7% outstanding.
There were also problems with serious case reviews - highlighted last month by Ofsted - with 29% found to be indequate.
Ms Gilbert denied that inspections were "box ticking" exercises - and rejected claims that the expansion of the watchdog had meant that it had lost its focus.
The annual report's observations on schools highlighted strengths and weaknesses.
"The overall picture is positive," said Ms Gilbert.
There were twice as many outstanding schools - and half as many inadequate - compared with four years ago, said Ms Gilbert.
This means that about 30% of schools are in the satisfactory or inadequate category.
But the report identified concerns over schools - including weaknesses in teaching that have a particularly detrimental impact on children from deprived backgrounds.
"There is too much that is mediocre and persistently so," said the Ofsted chief.
There were also warnings that inspectors had found five Academies to be inadequate and five satisfactory - out of a total of 30 inspected this year.
The report also had concerns over the standard of English and maths in primary schools.
"With the demise of the national strategies, it is vital that the importance of the acquisition of good basic skills is not diluted," said Ms Gilbert at the publication of the report.
Schools Minister Vernon Coaker said national strategies for English and maths had worked but that it was time to "move away from this top-down approach".
On the quality of teaching, Mr Coaker said there was "no excuse for any child not getting the best teaching possible - you only get one shot at education".
"The new Licence to Practise scheme puts teaching on a par with other high status professions. It will drive up teaching quality, by giving staff the support they need to improve, and weed out underperformers."
Shadow Schools Minister, Nick Gibb, said far too many children were being let down by the quality of education on offer.
"The problem of literacy in primary schools is holding back thousands of pupils, especially those from poorer backgrounds, and sows the seeds of truancy and disruptive behaviour later on.
"We have outlined plans to raise the status of teachers by increasing teacher quality," he said.
"Under a Conservative government the entry requirements would be increased and the quality of training improved."
Schools would be afforded more freedoms to pay good teachers more.
Liberal Democrat spokesman David Laws said: "It is shocking that after more than 12 years of a Labour government, nearly a third of schools are not providing a good education.
"This government's failure to drive up standards in all our schools will be one of its lasting legacies."
He added that Ofsted risked being diverted from its core purpose by the child protection agenda.