The SNP claimed East Dunbartonshire Council had over-estimated the value of land it could sell for housing as part of the £67 million plan.
The party also accused the Labour-led administration of failing to look at the option of refurbishing schools rather than closing them and building new ones.
Parents joined the criticism of the council, arguing that in some cases land values had been under-estimated.
The attacks come after the release of documents under Freedom of Information legislation showing estimated values for land currently occupied by schools, as well as refurbishment costs, savings from closures, and required borrowing.
In December, councillors voted to go to a preliminary public consultation on a set of plans for school closures and mergers which currently affects 24 out of the 36 primaries.
East Dunbartonshire says it needs to reduce its number of primary schools because they are operating below their capacity and are in poor condition, with council leader Rhondda Geekie stating: "Doing nothing is not an option."
However, parents are angry at the prospect of losing their local schools and do not feel all the options have been considered.
Ian Mackay, leader of the SNP group at East Dunbartonshire, said: "In our view, the amount of money the council expects to get from selling off some of these sites has been over-exaggerated.
"They are looking at borrowing significant sums of money already to fund this project which the council will be paying back for many years to come.
"The council needs to go back to the drawing board and make a proper assessment of both the values of land they could sell and also the cost of refurbishment because they may end up having to borrow a great deal more."
However, David Bauld, a parent from Gartconner Primary School, believes the council could get more money for some sites.
"The disposal values are all set with the view that 25% of the land would be used for affordable housing, but the council could meet its targets on one or two sites alone and the prime land could be sold at a much higher value," he said.
Niall Campbell, a parent from Baldernock Primary, added: "Baldernock is getting sold for £250,000 and Gartconner for £100,000 while the Bearsden site is only valued at £1 million. It just doesn't make sense."
The council agreed the quoted values were conservative estimates, which included a presumption that 25% of any residential development on the sites would be social housing.
A spokeswoman also stressed that the borrowing referred to was the maximum possible based on the savings generated from closing schools. For every £100,000 of saving, the council can borrow approximately £1.5m.
The estimated refurbishment costs are based on figures for five schools, which have been used as a template to work out a rough cost per square metre.
"A comparison of reference costs for refurbishments and new builds shows that we would expect refurbishment to cost up to 80% of the cost of new build, and not deliver the same benefits," she added.
Although the Scottish Futures Trust will fund refurbishments, it will only provide one-third of the cost, rather than half under a new build, she said.
Gordon Currie, the council's head of education, said: "These are preliminary working estimates from early in the process.
"As has been said many times, investing in our school buildings is fundamental to improving education even further in East Dunbartonshire.
"We need to make sure that our high standards of education are sustainable in the long term, and we will consider all the options."
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