The gloomy verdict by heads came as education minister Jane Hutt admitted extra cash was needed. The minister gave evidence last week to the Assembly government's finance committee, which asked whether the scheme is being adequately funded.
The committee met the day after 100 primary heads lobbied Assembly members across Wales. Around 40 heads also met with the minister to protest.
It was a high-pressured week for Ms Hutt, who took time out to visit Cardiff's closure-threatened Llanedeyrn High School and became embroiled in another contentious issue - the human cost of school reorganisation.
The foundation phase (FP) roll-out - due to begin in September for three to five-year-olds - has been extended from three to four years. But Ms Hutt said she was still pushing for more money from September 2009.
Last month, Ms Hutt announced an extra pound;5 million for pilot and early start schools. But the Welsh Local Government Association (WLGA) estimates another pound;13.25m is needed for the first year alone.
Sue O'Halloran, chair of the National Association of Head Teachers Cymru and head of Garth Primary in Bridgend, said many heads still did not know how much money they would receive in September. "As leaders we are in a vulnerable position," she said. "My school is going to be inspected in the autumn term and I need four extra staff members."
Iwan Guy, acting director of NAHT Cymru, said it would be even harder for Welsh-medium schools. "We are not aware of many Welsh-medium training providers. Nobody can say they didn't know it would be expensive as we raised these concerns five years ago."
Ms Hutt said robust data was needed to get additional funding and blamed local authorities for not providing information on staffing.
But the WLGA and the Association of Directors in Wales (ADEW), which gave evidence at the committee, disagreed.
Dr Chris Llewellyn, WLGA director of lifelong learning, leisure and information, said the budget should be based solely on pupil numbers. "Many good schools have been anticipating the roll-out and have put some provision and staff in place," he said."Unless we agree with the principle of pupil funding then they will be penalised."
David Hopkins, chair of ADEW and director of education and social services at Caerphilly council, said it was important schools could stick to the 1-8 staff to pupil ratio.
He said classes with between nine and 15 pupils should be eligible for funding to afford more staff.