Education Secretary Michael Russell has denied that the Scottish government is abandoning promises on education.
Labour has called for an emergency statement to the Scottish Parliament on the government's education policies.
Mr Russell offered councils a deal on speeding up its drive to cut class sizes through the scaling back of a commitment to give free school meals.
The move came as the Scottish government admitted it would not meet targets for smaller primary school class sizes during this parliament.
Opposition parties said the SNP had now scrapped pledges on both class sizes and free school meals.
New Education Secretary Michael Russell has offered councils flexibility on introducing free school meals to help them implement the class size policy.
The education secretary said the deal was a "realistic view of getting something done in very tough times".
Labour said two SNP manifesto commitments had been scrapped.
The government insisted cutting class sizes was vital but had to be paid for amid the current public spending squeeze.
Scottish Labour leader Iain Gray said: "The first minister told the people and Holyrood that he would deliver class sizes of 18 or less for primary 1-3. He also said he would provide free school meals for the same group.
Ministers said the offer would deliver maximum P1-P3 class sizes of 18 by August 2010 for 11,000 extra pupils.
This would mean 20% of children in the first three years of school would be in classes of 18 or less.
In return for speeding up class size reductions, the Scottish government said it would give councils "the flexibility to prioritise free school meals for children in the most deprived areas", as part of progress towards universal free school meals.
I do not have the finances to do everything I want to do Michael RussellEducation secretary
Ronnie Smith, general secretary of the EIS teaching union, said: "We have been going backwards on class size reductions instead of forwards so something needs to be done.
"But we have serious questions about whether this will be enough for the purpose, whether it will be robust and enforceable, or will it be optional to local authorities to buy into this if they so choose."
Scottish Labour leader Iain Gray said the changes proposed to councils amounted to the "ripping up of both the SNP's education manifesto commitments and the concordat".
Mr Gray said: "The first minister told the people and Holyrood that he would deliver class sizes of 18 or less for Primary 1-3. He also said he would provide free school meals for the same group.
The government said cutting class sizes was 'vital' but had to be paid for
"This has turned out to be false and he must come to the chamber and explain himself."
"This has turned out to be false and he must come to the chamber and explain himself."
Tavish Scott, the Scottish Liberal Democrat leader, said Mr Russell had run up the white flag on every SNP education promise.
We have now, as was well stated in parliament yesterday, £800m less in terms of available public spending in the next financial year Alex SalmondFirst minister
And Conservative schools spokeswoman Liz Smith said the SNP was applying sticking plasters to its broken education pledges.
But Mr Salmond said class sizes were at a record low and that the SNP manifesto pledge on school meals had been met.
Mr Russell, who last week replaced Fiona Hyslop as education secretary, said cutting both class sizes and free school meals were "enormously important" but he said there were real difficulties for local authorities due to the funding cuts coming from Westminster.
He said: "Everyone in Scotland is well aware that the world has changed substantially since we signed the concordat with local government two years ago.
"We are very keen on seeing manifesto commitments filled but we have got to go in a planned way according to the resources that are made available to us," he said.
"We have now, as was well-stated in parliament yesterday, £800m less in terms of available public spending in the next financial year. One third of that applies to local government."
Pre-budget report
He said proposals now with the Convention of Scottish Local Authorities (Cosla) aimed to make "substantial progress" in a difficult economic environment.
Mr Russell added: "We need to achieve the class size reductions because there is real evidence that it works and is useful.
"I think the parents and teachers and others in Scotland will respect the efforts that the government, and I hope Cosla, will make in order to bring as many as possible class sizes in primary 1-3 down to 18 or less."
"Councils are moving forward on school meals. I am recognising what they are doing. I am asking them to do more in areas of deprivation and will continue to grow that policy."
He claimed the school meals pledge to extend entitlement to 40,000 more children was achieved more than a year ago.
The education secretary said the chancellor's pre-budget report revealed that Scotland would suffer funding cuts of £800m next year.
However, the government promised last year to provide free meals in the first three years of primary school.
He said: "I do not have the finances to do everything I want to do. No Scottish government minister does. That is the reality of a cut of £800m.
Some councils had complained that they did not have the money to implement the free lunches.
"I hope I am opening the door to Scotland's local authorities to say 'there is still a way of achieving this', we are just going to have to move a bit more slowly and a bit more cleverly.
Now, in return for speeding up class size reductions, the Scottish government has said it would give councils "the flexibility to prioritise free school meals for children in the most deprived areas", as part of progress towards universal free school meals.
"I am now waiting for Scotland's local authorities to tell me what they think of it."
Liberal Democrat leader Tavish Scott said: "Mike Russell's first act as education secretary has been to run up the white flag on every SNP education policy."
Murdo Fraser, Scottish Conservative education spokesman, said: "Michael Russell is being fundamentally dishonest in claiming that the SNP's class size policy has had to be ditched due to Labour's recession.
"It was back in summer 2007 that senior government officials were saying that the policy could not be delivered, long before the banking crisis."
