The federal coalition has committed to bringing in its own version of "Gonski" school funding reform should it win office in September.
Opposition education spokesman Christopher Pyne made the pledge in letters sent to state and territory leaders outlining the coalition's approach to school funding.
However, Mr Pyne's stance has been seized on by unions and Labor as a backflip from the coalition's previously stated position that the existing school funding system was working.
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As recently as Monday, Mr Pyne told Sky News: "I can give you this guarantee. If the coalition's elected, we will keep the SES funding model (the existing model) for two years, we will keep the current indexation rate".
Prime Minister Julia Gillard has given the states until June 30 to sign up to the national plan for school improvement (NPSI), also known as "Gonski" after its creator, David Gonski.
NSW Premier Barry O'Farrell has signed up to the extra funding and encouraged his Liberal colleagues to do so.
South Australia's Labor premier Jay Weatherill is expected to sign soon, with the ACT and Tasmanian Labor governments still in talks.
In his letter, Mr Pyne states "the coalition government does not intend to unwind the NPSI should there be overwhelming agreement between the states and territories on the NPSI by June 30".
"However, should overwhelming agreement not be reached, an elected coalition government would take an alternative approach to the implementation of the recommendations contained in the Gonski report," he says.
The letter says the existing funding arrangement would be kept for "at least one year" while talks were held on a new system, which could include a common per student funding benchmark and "sector blind" loadings for disadvantaged students.
Australian Education Union federal president Angelo Gavrielatos - who is organising rallies across the country on Saturday to step up pressure on the states - told AAP the coalition had backflipped on its previous position.
"Christopher Pyne has acknowledged for the first time that the current funding system is broken and needs to be reformed," Mr Gavrielatos said.
"The real test for Mr Pyne is whether he and Tony Abbott publicly state their intention to support the government's legislation."
The prime minister says her deal will deliver an extra $16.2 billion over six years to public and private schools above what they would get if the existing system continued.
But Mr Pyne argued it amounted to a spending cut of $325 million over five years.
Visiting a Sydney school on Wednesday, Ms Gillard said she expected to get a national agreement by June 30 but understood the state leaders needed time to work through the funding offers.
However, she said schools were demanding certainty.
"Schools are entitled to know what funding position they are going to be in, in 2014," she said.
Shadow treasurer Joe Hockey told the National Press Club in Canberra it was not possible to say what the coalition would spend on schools until well after the June 30 deadline as the government's figures were rubbery.
"When the numbers stabilise and the deals are done, or not done, then we can offer a credible well-funded alternative," Mr Hockey said.
The "community rallies for Gonski" will be held on Saturday in Brisbane, Melbourne, Adelaide, Hobart, Darwin, Alice Springs and Perth.
The Sydney rally will be held in Mr Abbott's seat of Warringah.
