Specialist sports colleges have high proportions of poorer children, while language schools tend to be those for better-off pupils and girls.
New figures obtained from the Department for Education and Skills suggest that specialist schools may be promoting class and gender stereotypes.
Other data show that girls' schools make up a disproportionately high number of those gaining languages status while boys' schools shun arts in favour of sports and maths.
Teaching unions and the Equal Opportunities Commission said the news was worrying.
The figures also show that - despite a claim by the Specialist Schools Trust in 2001 - the proportion of pupils in specialist schools receiving free meals has been at least 1 per cent below the national average over the past three years.
Since 2001, the number of specialist schools has grown from 700 to 1,469, while the proportion of their pupils who fit the economically deprived category has dropped from 14.2 to 13.1 per cent.
At language schools just 10.6 per cent of pupils were eligible for the free meals, while at sports colleges the figure was 17 per cent.
John Bangs, head of education for the National Union of Teachers, said the differences were an "indictment of the two-tier system". "This shows what happens when you tamper with the comprehensive system, as the specialist schools do," he said. "You get a return to the old divisions."
The National Association of Schoolmasters Union of Women Teachers said it would be calling on the Government to ensure that the criteria for applying specialist school status took on board issues of equality.
Chris Keates, NASUWT deputy general secretary, said: "Education should be about giving children the widest choice possible rather than encouraging them down specific routes.
"Such evidence of stereotyping is one reason why we have concerns over specialist schools."
An analysis of single-sex specialist schools also reveals that they may be picking specialisms that conform to gender stereotypes. Girls' schools outnumber boys' schools by 116 to 81 - but even when this is considered the figures appear unbalanced in several subjects. Girls' schools account for double the number of language colleges that would be expected while very few opt for the sports specialism.
Meanwhile only three boys' schools have gained arts status yet far more than the expected number picked maths and computing.
Julie Mellor, chair of the Equal Opportunities Commission, said the figures showed that schools needed to ensure they did not "reinforce gender stereotyping" because the choice of specialism could influence pupils'
decisions about their future careers.
However, the Specialist Schools Trust denied the figures suggested stereotyping.
Ian Turner, the trust's director of external relations, said: "Specialist status is about widening pupils' options and breaking down traditional barriers such as social stereotyping. The fact that there is such a large number of girls-only technology colleges, for example, clearly demonstrates this point."
The Government this week defended the expansion of specialist schools in response to an MPs' report in May which said there was little proper proof they offered value for money.
The DfES said there was "extensive evidence on the effectiveness of the programme across schools and across the country".