The role of executive principal is "legally iffy" and the position must be officially clarified, a heads' union has warned.
Currently only one person can be designated the head of a school, but in the management structure of many federations and academy chains, a school has both a principal and an executive principal, with the latter overseeing two or more schools.
In this situation, is it unclear who is ultimately responsible for the institution.
About 700 school leaders across the country currently hold the title of executive head or executive principal, despite the fact that the role does not exist in pay scales or official documentation.
The number is only likely to increase in the next year or two: academy chains and federations are at the forefront of Government policy.
For example, Sir Michael Wilshaw, celebrated executive principal of Mossbourne Academy in east London, has also served as executive head of the neighbouring Haggerston School.
Despite repeated calls from the Association of School and College Leaders (ASCL) for the School Teachers' Review Body (STRB) to legally recognise the position, no clarification has been forthcoming from ministers.
ASCL general secretary Brian Lightman said it is unclear which of the leaders has ultimate responsibility for the school and its students.
"Who is accountable? Who does the buck stop at? It's quite a big question, and one that definitely needs clarifying. It's an important model, especially with there being so many academy chains."
Mr Lightman called for clearer distinction between the "core", day-to-day duties of a head and the wider, strategic responsibilities of some executive principals.
"Someone like that isn't really a headteacher," he said.
Stephen Szemerenyi, ASCL's pay and conditions specialist, said that developments in recent years have "outstripped the capacity of the pay and conditions document to cope".
"Governing bodies can manage to appoint somebody (as an executive head), but only according to whatever charter they may have been formed under. I actually think it's legally iffy.
"In law, the head is responsible for everything. But what exactly is the executive head responsible for? If they could be called the chief executive, it wouldn't be a problem," he added.
Fiona Hammans is executive head of the Banbury Dashwood Schools Federation in Oxfordshire, which consists of a primary and a secondary school, each with a deputy head responsible for its day-to-day management.
She described the legal confusion over executive principals' roles and responsibilities as "ridiculous".
"The situation is changing so much and the legislation, and particularly the STRB, isn't keeping up with what is happening in schools.
"My deputies are called operational heads, so are known locally as headteachers, but they don't have the legal responsibilities," she added. "My role is very much more strategic, looking after the budget, development plans and targets."
A Department for Education spokesman said: "It's true that the job is not explicitly defined in the pay and conditions document. However, it does provide a framework for recognising and remunerating heads that lead more than one school and for adjusting other teachers' pay who are affected.
"We know that most executive heads' pay issues are negotiated successfully locally. We know that they will play an increasingly vital role in the school system over the coming years so we are happy to continue to discuss this with ASCL and others."