Newly-elected Lib Dem MP Jane Dodds has called for an urgent meeting with Powys council bosses about school funding.
She said it was a key issue raised by voters during the Brecon and Radnorshire by-election campaign.
A Lib Dem councillor claimed schools would not be funded for extra pupils in September, but faced warnings if they did not balance their budgets.
But council bosses said schools were only warned if they failed to explain how they would tackle ongoing deficits.
Ms Dodds won Brecon and Radnorshire back for the Lib Dems last week after the constituency was held by Conservatives for four years.
She said she wanted to meet the council's leader Rosemarie Harris and chief executive Caroline Turner, with school funding one of the issues she intended to raise.
Despite a review of school funding in Powys to make it fairer, the group which set the new rules said it was £5.5m short of what was needed.
Lib Dem group leader James Gibson-Watt said: "Schools are now in the ridiculous and damaging situation of having to reduce teacher numbers to meet the requirements of the warning notices, whilst at the same time having to admit more students imposed on them with no extra funding.
"This can only have a catastrophic effect on the education standards in schools."
Powys head of education Dr Alec Clark said school funding was set according to the number of pupils in place the previous October.
He said the council "remains committed to working closely with schools and governing bodies to ensure appropriate education provision".
"Individual circumstances of schools are fully considered before any formal action is undertaken," he said.
"The local authority remains committed to working closely with schools and governing bodies to ensure appropriate education provision."