Heads are worried about the practicalities of the Diploma
Head teachers are warning of "doubts and uncertainties" that could undermine this year's introduction of the Diploma qualification in England.
The National Association of Head Teachers supports the Diploma in principle, but has a list of questions over how it is to be introduced.
These include funding, transport, child safety checks on workplace training staff and fears over computer systems.
The Diplomas will be academic and vocational exams for secondary school.
The NAHT heads' union is calling for much more "clarity" from ministers over the new secondary school qualification.
The first wave of Diplomas are to be taught in schools from this September - and they have been put forward as possible alternatives to GCSEs and A-levels, studied from the age of 14 through to leaving school or college.
But the NAHT, which is gathering for its annual conference in Liverpool, is highlighting heads' worries about the practicalities of implementing the reform.
Mick Brookes, the union's general secretary, says that heads are supportive of the idea of the Diploma, which will seek to bridge the divide between academic and vocational qualifications.
But he warns that "with systemic change of this magnitude, there are bound to be doubts and uncertainties".
The heads have worries about how transportation will work for pupils who have to be moved between different schools - in order to be able to access the range of options from the Diploma.
There are also concerns about the element of work experience - and the practicalities of child protection checks on staff in industry who might be working with Diploma pupils.
The heads are cautioning that the use of CRB checks will discourage businesses from wanting to participate.
There is also a question over the type of work experience that will be available to pupils with special needs.
The union also claims that there has not been enough testing of the IT system which head teachers say will be "essential" to the Diplomas.
If the Diploma is not a success and fails to motivate youngsters, the union has concerns that it will damage the plan to raise the age at which youngsters leave education and training.
It says that without a successful vocational route there is a "real threat" of "youth offending teams escorting young people back to school or college premises at the age of 18 years".
"School leaders have the scars to show for policy change and development without sufficient thought being given to issues of capacity and sustainability," says the union.
The Diploma project has faced a series of doubts over its implementation.
The Association of School and College Leaders has warned that confusion over its complex structure and a low take-up rate could doom the qualification to second-class status.
Apart from competition from the well-established A-level, the new qualification could also be rivalled by another proposed exam, called the Pre-U, aimed at stretching pupils planning to enter higher education.
Schools Minister Jim Knight defended the Diplomas as "a massive reform which will offer exciting new choices".
And the minister responded to some of the concerns of head teachers.
"The rules for CRB checks for Diplomas are the same as for any other work experience, are not bureaucratic and will be familiar to schools and colleges," says Mr Knight.
"The IT project for Diplomas is absolutely on track and will be ready well ahead of when the first Diploma exam components are awarded.
"On funding, we are making a major investment in the capacity of schools and colleges to deliver Diplomas, with schools getting on average an extra £1,000 per pupil per year taking Diplomas at Key Stage 4."