Controversial plans for two new academies could create a two-tier system that leaves some pupils with no opportunity to study a more challenging curriculum, say campaigners.
The proposals - which will see two secondaries in South Gloucestershire merge - are at an advanced stage, despite opposition from staff, parents and pupils, as well as union representatives who say it will leave the schools isolated.
The Ridings High School, in Winterbourne - which has some of the best GCSE results in the area - has been asked by the Department for Children, Schools and Families to sponsor the new Ridings Academy, due to open this September. The second academy, King Edmund, will be formed from King Edmund Community School in nearby Yate, where pupils do not perform as well.
But while The Ridings' curriculum will feature challenging qualifications such as the middle-years version of the International Baccalaureate, International GCSEs and a focus on maths and modern foreign languages, at King Edmund pupils will concentrate on vocational options in science, health and sport.
The schools' heads say this is because it will take time to organise a joint curriculum for both schools and that the admissions processes will remain the same.
But NUT officials say the proposed structure will not give King Edmund enough financial or governing independence.
"There is a danger this will create a grammar school in all but name," said Richard Brown, the NUT's South Gloucestershire secretary.
The initial consultation ended last year. Notification of closure for the two schools was published this week. Formal agreement for the federation could be given in the spring.
The schools will have one governing body and a chief executive principal, supported by a head of academy at each site. Both will open in September in their existing buildings, but staff and pupils from King Edmund will move into new and refurbished buildings from 2012.
Improvement work at The Ridings - made possible by the academies plan - will also start this year, with completion planned for 2016.
Only around 200 reponses were received to the consultation, the majority from people connected with The Ridings who were against the proposals. Some 55 per cent said no, and 29 per cent yes, with 15 per cent undecided.
Roger Gilbert, head of King Edmund, and Robert Gibson, head of The Ridings, told The TES that differences between curriculums at the two schools will narrow in the future, but at present they want to preserve their specialist status.
"There is good work going on at both sites and we don't want to lose that," Dr Gibson said. "We have ambitious plans for raising achievement at both sites, but the academies will be independent and they will both have their own budgets allocated. This will allow both to innovate. We were surprised by the level of responses - we thought it would be considerably higher."
Mr Gilbert said he was happy with the level of autonomy at King Edmund: "We believe both schools will keep their distinctive ethos," he said. "Our key area will be science, but one subject we have identified as needing improvement is languages, so we will work in partnership with The Ridings in that. The Yate community has been overwhelmingly supportive."
John Bangs, NUT director of education, said the proposal could hinder the development of both schools as it would leave them isolated.
"The aim seems to be aspiration and collaboration, but this becomes much harder and more complicated with an academy," he said. "I don't know why they didn't just become voluntary controlled schools or set up a 'half' or 'soft' federation."