Heads in Wales will today call on the Welsh Assembly to provide the money to back its plans for a distinctive education system.
Chris Howard, Welsh president of the National Association of Head Teachers, said: "It's time to start looking at practical realities."
He welcomed many of the policies introduced by the Assembly since devolution in 1999, including proposals for a play-based foundation stage for three to seven-year-olds, a Welsh baccalaureate and a more varied 14 to 19 curriculum.
But Mr Howard, head of Lewis school, Pengam, will tell the NAHT Wales conference, held near Cardiff today, that they can only be implemented if there is sufficient cash. "There are huge questions to be asked about overall levels of funding. We have grave reservations that we will be going down a road that has not been properly costed."
Proposals for a broader, more skills-based 14 to 19 curriculum will extend the range of courses and help schools work closely with colleges and businesses. Mr Howard said that heads need to know there is funding for tje changes as well as for the Welsh bac, being piloted in 18 schools.
The NAHT now plans to form a joint working committee with the Secondary Heads Association in Wales, to look at funding for the 14-19 curriculum and offer advice to the Assembly.
Similarly, Mr Howard believes that the Assembly needs to reconsider the practical implications of its new foundation stage. He said: "A play-based curriculum demands more classroom space. But in Wales we're encumbered by historic buildings that are not easily adaptable. The Assembly needs to look at dealing with these issues."
Mr Howard welcomed the less prescriptive approach to early-years teaching but called for more detail on how infants' achievements will be monitored.
"There is a legitimate concern that we don't throw the baby out with the bathwater. Literacy standards for six to seven year-olds have to be maintained."
He also wants to talk to Welsh education minister Jane Davidson about the workload agreement, to which the Assembly is a signatory. "At the moment, the implementation of the agreement in Wales looks like a dog's breakfast of missed opportunities," he said. "There's a real threat that it will be strangled at birth when heads realise they don't have the resources to move forward."
Mr Howard's demands are backed up by research, which shows that Welsh education is significantly underfunded. The research, conducted by David Reynolds, professor of education at Exeter university, shows that it suffers at the expense of other services, such as health. Next year, the annual per-pupil budget in Wales is likely to fall below its English equivalent for the first time since 2000.
Welsh schools also receive less devolved funding from local authorities, receiving only 80 per cent of the grant available, while in England they get 92 per cent.
The report states: "Schools in Wales may be several hundred thousand pounds poorer, in terms of available resources under their control, than those in England."