England's grammar schools say proposals for a new school diploma "should be firmly rejected".
Widespread changes in secondary schooling are in the offing
The National Grammar Schools Association said there was much to commend in the plans from the official taskforce headed by the former Ofsted chief, Mike Tomlinson.
They like the idea that "unacademic" youngsters should be identified early and that the most able need to be stretched more.
But they say the proposals are based on the principle of "all must have prizes", making it impossible for grammar and independent schools to show their pupils' "outstanding achievements".
The association, representing the 164 grammar schools, agreed in its response to Mike Tomlinson's consultation that the present exam system was "in a sorry state".
But it said: "There is a grave danger that the Tomlinson proposals will further the egalitarian cause at the expense of academic standards."
It said his interim report, published in February, revealed "a new crisis in education, which has been caused largely by comprehensivisation and grade inflation".
Mr Tomlinson proposed a diploma with four levels of difficulty, up to the equivalent of the present A-levels.
It would have three parts: a core, including numeracy and literacy - not to be confused with GCSE maths and English - "main learning" and a "personal challenge".
At the higher levels there would be more academic or more vocational diplomas.
'Equality of results'
The grammar schools' association suggested the goal of the qualifications authority and government-appointed groups such as Mr Tomlinson's was to get "equality of results" from the education system.
"The conflict between equality of opportunity and equality of results is at the heart of the present debate, but fails to get a mention in the Tomlinson report."
It said equality of results "has a pleasant ring to it, but what is at stake is whether the equality ought to be achieved by lifting everybody up to the standard of the highest or holding everybody down to the level of the lowest".
"The answer to this question is obvious, but the real issue is whether it can be achieved.
"The current prevalence of the radical/sociological/Marxist concept of equality of results is alarming."
It added: "There is a grave danger that the Tomlinson proposals will further the egalitarian cause at the expense of academic standards."
The association's response has itself drawn fire from the second biggest teachers' union, the NASUWT.
Its acting general secretary, Chris Keates, said the association was being "needlessly alarmist and misleading".
"Everyone concerned for the education of all our young people wants to see a system which allows students to achieve the highest possible standard of education," she said.
"Anyone who has read the Tomlinson proposals knows that suggestions that it seeks to drag down academic standards are pure nonsense.
"Instead of raising fears about agendas that do not exist, the NGSA should spend its time more productively by reflecting on how it can work constructively with those involved in the Tomlinson review to create a well-funded, high-quality and equitable education system in the years to come."