There must be a three-year trial of the vocationally led 14-19 learning pathways before it becomes law, headteachers said this week.
They gave their verdict as an Assembly government report expressed disappointment that officials could not come up with a figure for the full cost of implementing the new measure.
A review of the proposed measure, which will make learning pathways compulsory by next September, has been released by a cross-party finance committee.
The report says that considering the looming recession and troubling financial times, officials should come up with a final figure.
It goes on to recommend that the measure's passage be halted until a reliable estimate, not based on assumption, is arrived at.
The new curriculum offers more subject choice to 14-year-olds, and learning coaches - or mentors - will help them to choose the right subject paths.
It is hoped that more vocational choice, made possible through shared timetables with FE colleges, will prevent young people from dropping out of school.
Many schools are already introducing learning pathways, and all schools will have to offer 30 subject choices by 2012 under the proposed measure.
But heads say the new measure is being implemented too soon, and that the initiative is underfunded - Pounds 32.5 million has been awarded in the 2009-10 draft budget.
They warn there could be a repeat of the foundation phase funding fiasco for under-7s, but that at least had the benefit of a three-year well- funded pilot.
An extra Pounds 60m was found for the play-led learning scheme after officials admitted it lacked funding earlier this academic year.
Gareth Jones, secretary of heads' union ASCL Cymru, said he "regrettably" endorsed the conclusions of the finance committee, despite supporting the measure's aims.
"We propose that the implementation process should follow the example of the foundation phase and involve a fully funded pilot stage," he said. "This would be the only way to realistically assess the full costs and barriers to implementation in all schools and colleges at a set date."
But Dr Philip Dixon, director of teachers' union ATL Cymru, said the report's findings raised serious questions about the funding of key education policy in Wales, particularly after new research showed significant underinvestment in Welsh education compared with the rest of the UK.
TES Cymru revealed the findings of education expert Professor David Reynolds in a special edition earlier this month. He concludes that, taken as an average, 6.4 per cent less has been spent on education in Wales compared with the rest of the UK since devolution.
Dr Dixon said: "As the Assembly government has not disputed the figures given by Professor Reynolds, we can only assume that they accept the funding gap exists and is roughly of the magnitude he suggests.
"Even more disturbing are these latest concerns from the finance committee. It seems the government is not just funding below the English level, but also not funding sufficiently for its own policy agenda."
David Reynolds, professor of education at Plymouth University, said the finance committee appeared to be worried that there was not enough money budgeted to cope, especially as public expenditure was about to get very tight.
"It will be very difficult to claw back from other services," he said.
An Assembly government spokesperson said it would respond to the report's findings shortly.
More learning pathways, page 5
Leading article, page 36.