City academies may not be the best places to put money into for people wanting to spend £2m to help education in England, a charity says.
Questions have been raised about the new buildings
"Academies are a risky investment: they can and do fail," said a report from New Philanthropy Capital.
The charity, which advises donors on effective giving, suggested instead helping victims of bullying or children with special educational needs.
The government said it was wrong to say academies cost more than other schools.
The charity's report, On your marks: Young people in education, comes amid concerns about the academies programme.
These have been prompted by allegations, linked to the Labour Party "cash for honours" issue, that sponsors could expect titles.
Prime Minister Tony Blair, at his monthly Downing Street press conference on Monday, praised the sponsors of his academies for their "money, time, effort, energy and years of hard work".
"Insofar as the honours reward people who contribute to society, contributing to the education of disadvantaged kids in the inner cities is about as good a contribution to society as I can think of," he said.
The New Philanthropy Capital report did not refer to this ongoing row. But it said the emphasis on academies had led to "a narrow view of giving, ignoring many of the other options".
For £2m, sponsors have the opportunity to create a new school - defining its ethos and getting a controlling say in its running, while the government pays the running costs.
The report said the buildings were "extremely expensive".
"State-of-the-art buildings look good and pupils and teachers are very proud of them, but there is scant evidence of the link between this capital investment and pupil attainment."
It said: "There is too little evidence to assess conclusively whether or not academies are a good investment for donors.
"But there is enough evidence to raise doubts as to their cost effectiveness. Academies show mixed results for their pupils."
It said the role of charities in education was not well understood.
Donors who wanted to support young people had "a limited menu of options".
"Government-backed programmes were often emphasised to the detriment of other, less glamorous options, such as supporting after-school hours activities, or projects that combat the effects of bullying."
On bullying, for example, it said: "The charitable sector is the acknowledged expert" - before outlining the work of a number of organisations, with costs.
"For £2m, donors could train 2,000 teachers, reducing bullying in their schools by up to 40%."
Teachers' unions have argued that the money the government spends on setting up academies - typically £23m apiece, according to the Commons education committee - would be better spread more widely.
The New Philanthropy Capital report compared this with the £16m to £17m for a conventional school, noting: "The £8m discrepancy between these figures could pay the wages of 276 experienced teachers for a year."
But the Department for Education and Skills said they cost "exactly the same as similar-sized schools".
A spokesman said: "Support from sponsors is buoyant and we are already halfway to our target of having 200 academies open or in the pipeline by 2010."
One sponsor, Lord Harris of Peckham, said: "This report does not hold water - sponsors like myself strongly support academies because they are beacons of success in their communities and transform lives for the better."