Ministers have been accused of "misleading" the public over the city academies programme in England.
City academies are housed in new buildings
An evaluation of the first 11 privately sponsored academies by consultants PricewaterhouseCoopers this month gave a mixed picture of their success.
But the government's response took "the most favourable view possible", The Education Network (Ten) said.
However, a government spokesman said academies were making a "significant difference" to classroom culture.
'Open and honest'
The Ten report said the academies scheme was so expensive - at an expected £5bn - that it was "essential" for the government to evaluate it properly.
The organisation, which advises local authorities, called for "a far more open and honest approach to the presentation of information".
Academies, costing an average of £25m to build, are intended to turn around the fortunes of failing urban schools.
The Department for Education and Skills (DfES) said the PwC evaluation showed three quarters of pupils felt behaviour was better at their academy than its predecessor.
But Ten said the figures were inaccurate and "misleading".
"In fact, PwC reports that 69% of pupils disagreed with the statement that 'my behaviour is worse now than before I came to this academy', which is not the same thing at all," it said.
"Such manipulation of the data is disingenuous and the removal of those replying 'don't know' also presents a misleadingly favourable impression."
Ten also said apparent parental endorsement, based on 10 academies, relied on a survey conducted before some had even opened.
It called for "a fundamental change of approach" to the academies debate, adding: "It doesn't help anybody that it is shrouded in secrecy, lack of hard data, disingenuous interpretation of the data that is selectively available etc.
"That is bound to create the polarised positions that are now being taken up by many."
The first academies opened in 2002. Under the scheme, private sponsors put in up to £2m and in return appoint most of its governors.
The government funds the rest of the start-up costs.
Academies are regarded as "independent" schools within the state system and 200 are planned to be built or under construction by 2010.
A DfES spokesman said: "The independent [PwC] evaluation shows the strong support of parents towards academies and finds that there is 'a significant difference in the learning culture in new academies, compared to their predecessors'.
"The report is part of a five-year study that will ensure comparisons with other schools are available."
He added: "We cannot afford to wait, condemning a generation of pupils in some of the toughest parts of the country to the failed approaches used in the past."