Head teachers have rejected as "naive" a report which suggests almost a million pupils in England are in unsatisfactory schools.
Dr Dunford said the report had manipulated statistics
The National Audit Office report said 1,557 schools were found to be poorly performing and 242 "failing".
But the Association of School and College Leaders accused the public spending watchdog of an "injudicious use of statistics".
The association said the report had led to "false" newspaper headlines.
General Secretary of the ASCL, John Dunford, said: "The report is naive in failing to recognise that 25% of schools will always be in the bottom quartile.
"This is a very weak report and I'm very disappointed in the conclusions it has produced - these conclusions are entirely inappropriate."
Dr Dunford said the report was not helpful in raising the morale of schools and staff.
"Schools will find this very disconcerting at a time when they are working harder than ever to raise standards," he said.
The NAO report also highlighted the problem faced by many schools in recruiting head teachers.
Of the schools that advertised for a head teacher in the 2004-05 school year, more than a quarter of primary schools and a fifth of secondary schools did not have a permanent head at the time.
While critical of much of the report, Dr Dunford said it was good to highlight the shortages of school leaders.
"This is a very worrying statistic and demonstrates the need for a much clearer understanding of the current pressures on school leaders, which are preventing people from applying for leadership posts in sufficient numbers," he said.
Mick Brookes of the National Association of Head Teachers agreed with his comments.
He said recruitment was clearly a key issue.
Being a head was "a fabulous job" but there were two big problems with doing it.
It was high risk.
"Everybody knows if Ofsted comes in and finds you wanting, one of the first things that happen is that the head goes," he said.
This was particularly so in view of the "very aggressive" words in the government's White Paper on education.
Secondly, heads were overburdened by government initiatives - "we've counted 65 on the go at the moment".
"It's a great job but they are making it impossible to do and we have got to do something about that," he said.
The chief executive of the General Teaching Council for England, Carol Adams, said outstanding head teachers and teachers were the key to transforming poorly performing schools.
"To attract and keep the best teachers in challenging schools requires sustained investment in pupils and a package of incentives for teachers," she said.
"These pupils deserve - above all else - talented teachers who stay and who are motivated to become heads."
William Atkinson, the headmaster credited with turning around the once-failing Phoenix High School in London, said he was not surprised so many schools were without heads.
"I think you need to look at the incentives to work in these challenging schools and I think we need a salary premium, not only for the leadership but also for all teachers working in these institutions," he said.