Meetings have been hastily arranged following the decision of heads in the Vale of Glamorgan, alongside colleagues in Cardiff last week, to make a stand against a lack of funding for the strategy.
They say government funding for the scheme, which becomes statutory for all infant pupils this September, is "too little, too late", and schools face making staff redundant to compensate.
They also believe a staffing shortage - it is estimated that 3,000 more teaching assistants are needed - will make it impossible to achieve the recommended staff-to-pupil ratio of 1:8 for under-5s.
Signs that the government is taking the threat of a boycott more seriously after months of protests emerged this week after it was announced that a group of "key partners" would be parachuted in to investigate concerns. The panel will report its findings in early May after listening to complaints.
The 20089 academic year is the most important for Welsh education in a decade, but it looks set to be turbulent, with national strike action over pay likely to continue and growing discontent over funding for key initiatives in Wales.
TES Cymru first reported threats of a boycott last November, as well as possible strike action by the NASUWT Cymru teachers' union that was later averted.
Unions and the Welsh Local Government Association (WLGA) united before Christmas to lobby the government for more cash in their budgets. But they were left disappointed by cash awards which local authorities were finally made aware of by the government in February, leaving heads warning that they would have huge budget shortfalls.
Kirsty Williams, education spokesperson for the Welsh Liberal Democrats, said: "While everyone supports the FP, I have severe concerns that the government has not put sufficient time or money into this flagship initiative."
She said the government now also accepted that there would not be enough staff available for the roll-out of the FP in September.
"They are now saying they will phase it in over four years and schools should work towards achieving the adult-pupil ratios," she said.
The government has always maintained funding is adequate. A spokesperson said this week that more than pound;107 million had been allocated to the FP as a specific grant, agreed by the Association of Directors of Education in Wales and the WLGA, for the next three years.
But David Griffiths, head of Peterson-Super-Ely Church of Wales Primary in the Vale of Glamorgan, said some schools would be left short. "Shortfalls could be as much as 50 per cent," he said. "One school will have an allocation of just over pound;30,000 when more than pound;100,000 is required."