Welsh heads called for the end of the per-pupil funding gap between Wales and England and demanded clarity in the way resources are allocated to schools at last week's NAHT conference in Liverpool.
Anna Brychan, director of the heads' union NAHT Cymru, said members in Wales were "desperately concerned" about the lack of clarity over the severity of cuts facing public services and what it may mean for schools and pupils.
A recent Wales Audit Office report suggested that the Assembly government's budget may lose pound;3 billion over three years.
Ms Brychan said: "We know that painful cuts are coming - every party fighting the UK general election has repeatedly told us so.
"School pupils in Wales are already living with a pound;527 per-pupil funding deficit compared to fellow pupils in England. We're starting from a low base and are worried about falling further behind."
Ms Brychan also took issue with claims that schools are sitting on excessive balances that could easily be released to combat anticipated funding problems over the coming years.
"The notion that there is a huge amount of cash out there sitting idle in school bank accounts is nonsense," she said.
"Often the school balance picture is clouded in Wales because of the way balance amounts are reported."
David Griffiths, president of NAHT Cymru and head of Peterson Primary School in the Vale of Glamorgan, proposed a motion calling for the end of the funding disparity and demanding that resources allocated to schools are transparent and easily tracked to school level.
Education Minister Leighton Andrews has said that the schooling system in Wales must become "smarter and simpler", and has ordered a major independent review of education spending.