Concerns are growing about the pressure on academy heads as four have already stepped down from their posts.

The turnover of principals at the flagship schools has appeared unusually high because only 12 academies have been opened, the majority for less than a year.

The Secondary Heads Association said that departures by other senior staff at the state-funded independent schools were also worrying.

John Dunford, general secretary, said: “We are very concerned at the rate of turnover of academy heads and other members of their leadership teams.

There is a misplaced expectation that these schools, which may have been in difficulties for many years, can be turned around overnight. Ministers should be more patient.”

The Labour government has put academies at the heart of its plans to revamp secondary education and has pledged that 200 of the state-funded independent schools will be in development by 2009.

Many academy principals are head-hunted for the posts and earn six-figure salaries.

The academy which has the highest turnover of heads is the Greig academy in Haringey. After Graham Horsewood resigned as principal last year, David Triggs was brought in to act as executive principal, and Richard Bassett, the school’s second official head, will be replaced next term by Paul Sutton of Broomfield school in Enfield.

Frank Thomas, head of the Capital city academy in Brent, also resigned this summer citing personal reasons.

Richard Coupe, the original principal of the King’s academy in Middlesbrough, was demoted to senior vice-principal and left before the school opened last year.

Academy heads said they felt under a lot of pressure and feared that some people’s expectations could be too high.

Peter Crook, head of the academy at Peckham in Southwark, said: “It’s the hardest job I have ever done - it is certainly not a job for a first-time head. What needs to be remembered is that it is going to be a marathon not a sprint.”

The view was shared by Ray Priest, head of the Bristol academy. “It’s a high-pressure job but that is what I am paid to do,” he said. “You are very much aware that you are in the spotlight.”

Tom Widdows, principal of the Bexley business academy, has faced false rumours that his pound;31 million school failed an inspection and that his job was under threat. Mr Widdows said that, although the full report would not be published until next month, inspectors had told him the academy was a “rapidly improving” school where the ethos had been transformed.

They had also praised the academy’s leadership, he said, and described improvements in its examination results as “remarkable”. Although only 21 per cent of pupils at the academy gained five A* to C grades at GCSE last year, the figure is a vast improvement on that of the school it replaced, Thamesmead community college, in which as few as three pupils achieved those grades.

A government spokesperson said: “We acknowledge that raising standards in schools which have a long history of poor performance is a difficult and challenging job, and that success is likely to take some time.”