Headteachers are earning annual consultancy fees of up to pound;50,000 on top of their regular pay for advising struggling schools, The TES has learnt.
Successful school leaders are generating the money while working solely in the state sector by charging up to pound;850 a day for their expertise. A number of heads have struck deals with their governing bodies, allowing them to work as private consultants and then share the profits between themselves and their schools.
The revelation raises questions about the rules governing salaries for heads who take on responsibilities at more than one school.
It also comes as schools prepare for their budgets to be squeezed in a bid to cut public-sector debt.
Ed Balls, Schools Secretary, has called for the system for paying executive heads, who take permanent charge of more than one school, to be overhauled. He wants extra money to attract top heads to take on federations.
However, headteachers are already earning considerable rewards for themselves and their schools by acting as advisers.
As headteacher of Greensward Academy in Hockley, Essex, David Triggs came to an arrangement with his governors to set up his own consultancy business, Empowering School Improvement.
Under the deal, any money he made from the consultancy was split between himself and the school. Mr Triggs has repaid his full annual salary to the Academy on three occasions, meaning that the school effectively received his services for nothing.
During his most successful year he made about pound;50,000 on top of his salary, charging a maximum consultancy rate of pound;850 a day.
Mr Triggs, who is now chief executive officer of the Academies Enterprise Trust, which runs three academies in Essex, including Greensward, likens the arrangement to doctors who combine NHS and private health work.
"Nobody thinks twice about that and I don't think there is anything wrong with it for a head," he said.
"I'd say to other heads, if you are going to do it, do it with the governors' agreement and be absolutely transparent about it."
Mr Triggs is now winding down his business because he says he no longer has time to combine that with his chief executive role.
Arrangements are "the norm"
But according to Sir Paul Edwards, the head of Garforth Community College, near Leeds, similar arrangements are "quite the norm".
"I can think of half a dozen off the top of my head," he said. "I would not be surprised if about pound;50,000 extra a year was the typical amount.
"It depends on the scale of the challenge, but I don't think that amount would be out of order."
Sir Paul, who as chief executive of the School Partnership Trust is involved in running five schools, said it was right that heads were properly rewarded.
"If they are not, where are we going to find these people?" he said. "Models that encourage schools to work together can be very cost-effective because you get economies of scale."
However, despite overseeing numerous schools, the fees that Sir Paul charges go straight into Garforth's budget and not to him.
At the end of the year, if he has hit various performance targets, some of that money can be used by governors towards a pay rise.
A similar system of repaying a head's school rather than the individual is used in the National Leaders of Education (NLE) scheme, coordinated by the National College for School Leadership.
There are more than 300 NLEs working in 250 schools across England. Their home schools are paid for the heads' time, but the individuals only get extra pay if they work overtime.
John Dunford, general secretary of the Association of School and College Leaders, said that higher pay was right as long as the deals that schools were signing up to were transparent.
"Headteachers who take on responsibility for more than one school are running a professional risk," he said. "It seems perfectly acceptable that different governing bodies come to different arrangements, as long as it's all done in the open.
"There is nothing to be ashamed of - these people deserve extra money, but public service demands a greater degree of openness than the private sector."