A teachers' leader says his union has hardened its opposition to government plans for 200 city academies.
Steve Sinnott: Pension changes are also troubling members
Steve Sinnott of the National Union of Teachers said academies would seek to keep out children who did not get good results or had learning difficulties.
A resolution to the union's annual conference calls for a campaign against government plans to have 200 of the independent state schools in England.
The government says the academies must take pupils of all abilities.
Mr Sinnott said those running academies were under "enormous pressure" to succeed.
"The easiest way in which you can make it appear that you are successful is by changing the children, not by changing what you actually do," he said.
"The children who are seen as most likely to depress the test results for the school will go elsewhere.
"You create a situation in which they become schools that are much more attractive to parents who have the higher aspirations, who have the skills to find their way around the education system."
Mr Sinnott was discussing with journalists the agenda for his union's annual conference at Easter.
The motion opposing academies describes them as "a form of privatisation of public schooling".
Academies are sponsored by outside groups - such as companies or church groups - which have a majority on the governing body though the government puts in the bulk of the funding.
Being independent, they can change such things as the curriculum, length of the school day and teachers' conditions.
In a separate motion about children with special educational needs, Mr Sinnott said: "It shouldn't surprise us that academies may be in a position whereby they may encourage some and discourage others with special educational needs."
A spokesman for the Department for Education and Skills said academies had to comply with the law on admissions and cater for children of all abilities.
"The code of practice doesn't allow academies to cherry-pick pupils," a spokesman said.
They were "local schools for local people".
A government-commissioned report on academies by consultants Pricewaterhouse Coopers, obtained by the Guardian newspaper, said researchers had found that middle-class families tended to benefit most from such schools.
The report looked at "charter" schools in the US which they said offered a "close parallel to academies in the UK" and found that improvements and innovation in teaching and learning had been "modest".
It also found that staff in the academy-style schools spent more time on non-teaching duties than their counterparts in state schools.
Other issues the NUT conference will be debating include pay and pensions.
It is aiming to hold a one-day strike, probably in April, over the government's plans to raise the retirement age for public sector workers from 60 to 65.
There is likely to be a resolution criticising the government's rejection of the Tomlinson plan for a secondary school diploma in England.
Mr Sinnott said there had been "a real consensus" behind the idea and he hoped it would be reconsidered after the general election.
He also was going to hold a formal meeting as soon as possible with Education Secretary Ruth Kelly, with whom he hoped there could be a "mature relationship".
Relations between government and union have been fraught in recent years, with ministers refusing to attend its conference and the union not signing up to the agreement on teachers' workloads in England and Wales.