Swine flu could force cash-strapped schools to close within weeks of the new term because heads cannot afford cover for absent teachers, it was claimed this week.
Heads told TES Cymru they would not have the financial resources to cope if even a small number of their staff were struck down with the virus, because their budgets are so stretched.
Last week, Paul Davies, Conservative shadow education minister, accused the Assembly government of "extraordinary complacency" over the potential impact of swine flu on schools after education minister Jane Hutt revealed that no financial contingency plans were in place to cope with an outbreak.
In a letter to Mr Davies, Ms Hutt admitted staff absences were likely to be widespread in a pandemic, that schools would face "some additional expense" finding cover staff, and that demand for replacements was "likely" to outstrip supply.
Mr Davies said: "School budgets are already stretched to breaking point, even before the prospect of higher costs as a result of swine flu.
"Without additional support from the government, a swine flu pandemic will place teachers under even greater strain."
Neil Foden, head of Ysgol Friars in Bangor, said many schools had been forced to spend their emergency budgets.
"Because the government has kept schools and councils so starved of cash over the last couple of years we have no financial contingency," he said. "If a pandemic does occur and a lot of teachers are off sick, I don't know how we would cope. Schools just don't have the financial flexibility."
Mr Foden said his 1,250-pupil school would be forced to close if 10 of his 80 teachers became ill at the same time.
Alan Toothill, head of Penyrheol Comprehensive, near Swansea, said funding cover for staff with swine flu, coupled with other long-term sickness cover, could prove "very expensive indeed". But he said he would be surprised if the government did not find the cash needed to keep schools open.
Heads of smaller schools said they would be under even more pressure.
Paul Welsh, head of the 210-pupil Padre Pio RC Primary in Pontypool, said: "If members of staff come down with any prolonged illness, that would have a huge impact on us. It would be a huge financial burden to the school and I don't know how we would manage."
But some heads, including Bethan Guilfoyle of Treorchy Comprehensive, said they would try their hardest to keep their schools open.
"I'm sure our staff would rally around and do their best in such exceptional circumstances," she said.
Cheryl Wheldon, head of Coedffranc Primary in Neath Port Talbot, said: "When we have the general flu in winter, it's nothing for us to have four or five members of staff off. We have to deal with that."
David Reynolds, professor of education at Plymouth University, said many schools in Wales were financially unprepared for a serious outbreak and that it could cost the average secondary tens of thousands of pounds in cover fees.
A spokesman for the Assembly government said accusations of complacency over swine flu were "inaccurate and irresponsible" as robust contingency plans had been in place for some time.
"We are keeping this situation under review and will provide schools with updated advice prior to the start of the new term," he said.
Meanwhile, England's boarding schools have reported that foreign student numbers are down because of concerns about swine flu.
At Rossall School in Lancashire, admissions officers said several students had pulled out of its summer school and about half a dozen boarders had decided not to attend next term.