Opposition parties this week threw out the Assembly government's spending plans for 2006, claiming ministers had got their spending priorities wrong - especially in education.
A lack of cash for small schools and classroom essentials played a large part in the minority Labour government's budget defeat.
Finance minister Sue Essex must now return to the Assembly with a revised budget taking into account opposition demands.
But Ms Essex insisted that any changes to spending would take funds away from other frontline services and that the opposition had failed to cost its demands.
The draft budget set aside pound;1,553 million for education and lifelong learning in 2006-7, up from pound;1,364m in the current year, and included significant increases in support for universities, early years and governors.
But opposition Assembly members claimed it was not putting enough cash into frontline education services and a fund supporting small schools, and failed to address the historic funding gap between Welsh and English universities.
Janet Ryder, Plaid Cymru's shadow education spokesperson, said more money should be given to small schools to save them from the axe. Wales has a far higher proportion of small schools, defined as 90 pupils or fewer, and falling rolls are adding to the pressures on LEAs to reduce surplus places.
Ms Ryder added: "Parents will hear of cuts in the classroom and wonder why we are being told that more money is being put into education."
She said the opposition's intention was to thrash out funding concerns, but not to include proposals for changes to the budget.
Liberal Democrat Peter Black, chair of the Assembly's education and lifelong learning committee, told members the amount of cash left over after local authorities had paid for teachers' salaries was not enough to fund other classroom basics.
Fellow Lib Dem Jenny Randerson said council funding had been squeezed and the capital budget for improving school buildings was "woefully inadequate". And other members said funding for new initiatives, such as free breakfasts for primary pupils, should be scrapped and the cash put back into schools.
Iwan Guy, acting director of the National Association of Head Teachers Cymru, said initiatives such as the foundation phase were welcome, but needed adequate funding.
"The workload agreement means money is tighter than ever and frontline education needs to be adequately funded," he added.
Gruff Hughes, general secretary of Welsh-medium teachers' union UCAC, welcomed the debate on the funding of small schools - but does not want the government's defeat to delay budgets for next year.
He said: "The future of small schools should be safeguarded, but new initiatives should be properly funded."
An Assembly government spokesperson said ministers would discuss the budget with opposition parties, Assembly committees, and "wider civic society", before presenting a final version before December 10.
"The government has been clear in setting out its spending plans, with targeted investment on frontline services and a focus on improving healthcare and investing in children," he said.
"We hope to continue with a radical agenda of change."