PRIME Minister Julia Gillard and Premier Jay Weatherill this morning signed the Gonski deal, which will deliver an extra $656 million over six years to South Australian school students.

Ms Gillard threw in an extra $90 million to sweeten the deal as she tries to get all the states on board before the June 30 deadline.

"I am proud that we are delivering this important investment in our children's future," Mr Weatherill said.

"These historic changes to education funding are about ensuring that every child gets what they need for a high-quality education - because every child is different and has different education requirements."

The Coalition has pledged to repeal the education funding reforms if it wins power at the September election.

Ms Gillard set a deadline of June 30 for the states to sign up, saying that would provide certainty to schools needing to set budgets.

Federal Education Minister Peter Garrett, who was in Adelaide yesterday, had to hose down concerns that deals were being done to secure the states' agreement.

Meanwhile NSW Premier Barry O'Farrell is now investigating whether his state was dudded after Ms reportedly trebled her offer to Western Australia.

NSW was the first state to sign up, and Mr O'Farrell said Ms Gillard had promised her offer to other states would not change.

"At the press conference, the Prime Minister clearly said the same deal that NSW was signing up to was available to other states and territories - no better, no worse," he said.

The complex federal funding model distributes Government money based on student need and disadvantage.

Opposition education spokesman Christopher Pyne said that the apparent deal with WA to get a signature seemed to be a "betrayal of trust".

He said: "The states should be very concerned that Labor is not dealing honestly with them or honouring agreements."

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