Serious concerns have been raised about the high turnover of headteachers at academies sponsored by the programme's biggest backers.
More than half of the principals at schools sponsored by the United Learning Trust have been replaced within two years of the academies opening. One headteacher - at Sheffield Springs Academy - served only one term in post after the school opened.
Eight principals at the Christian charity's 15 academies have left their jobs, leading to concerns about instability at schools and complaints that pressure is being put on heads to deliver results unrealistically fast.
John Dunford, general secretary of the Association of School and College Leaders, said: "The rate of turnover in these academies is extremely worrying and suggests either that the appointment process has shortcomings or, more likely, that there are unrealistic expectations of what the principal can achieve in a short time.
"You have a double whammy of the previous headteachers losing their jobs and the new academy head often quickly losing theirs, also because they are not fulfilling the sponsors' unrealistic expectations."
The concerns follow rapid changes to headteachers appointments at academies run by other sponsors.
Ruth Johnson, former principal of Oasis Mayfield Academy, in Southampton, stepped down at the end of last year less than a term after the school opened.
The academy had been hit by a series of problems, including the exclusion of 30 pupils after school property was damaged.
And both the chief executive and headteacher of the Richard Rose Central Academy in Carlisle, sponsored by businessman Brian Scowcroft, were forced out of their jobs earlier this year less than two terms after the school opened.
The school had been heavily criticised by Ofsted, but new chief executive Mike Gibbons has said that his predecessors were given insufficient time to improve the situation.
As revealed in The TES earlier this month, Fiona Cordeaux became the latest ULT principal to step down after a series of problems at Walthamstow Academy in east London.
Phil Hearne left Paddington Academy in west London at the end of last year after less than 18 months as principal. Barbi Goulding, its first principal, left the academy in May 2007, just a year after it opened.
Other ULT academies to have changed their heads are Lambeth, Northampton, William Hulmes' Grammar, Sheffield Springs and Sheffield Park.
Tim Hastie-Smith, named principal of Kettering Academy, which was due to open this September, agreed to step down before even taking up the job after it was revealed that he had employed a teacher knowing that he had filmed pupils having sex on a school trip.
A ULT spokesman said that taking retirements and promotions into account, only four heads had left to work for a different organisation.
He also pointed to an improvement in exam results at its academies. Paddington was the most improved academy in the country last year, with the proportion of pupils achieving five good GCSEs including English and maths, up from 25 per cent to 42 per cent.
Worries about the turnover at ULT come in the wake of Dr Dunford raising concerns about the overall rate of headteachers being replaced in all types of secondaries.
Figures released by his association earlier this year showed a five-fold increase in the number of heads being forced from their jobs in the past four years.