Pupils are having their options "narrowed" and "distorted" by the English Baccalaureate, a meeting at the House of Commons was told.
The meeting, called by Labour's Andy Burnham, heard some pupils who had begun their GCSEs were being switched mid-course to baccalaureate subjects.
A teachers' union warned schools were cutting non-baccalaureate subjects.
An education department spokesman said the English Bacc "opens up" academic subjects to a wider range of pupils.
Mr Burnham, the shadow education secretary, said choices about the importance of different subjects should not be "decided on a ministerial whim".
The English Baccalaureate - or "E Bacc" or "English Bacc"- sets out a new league table measurement for secondary schools in England, based on how many pupils achieve a good GCSE in English, maths, two sciences, a language and either history or geography.
A Department for Education spokesman said: "The English Baccalaureate opens up academic subjects to tens of thousands of pupils who wouldn't get the chance to study them otherwise.
"The E Bacc is not compulsory - it leaves plenty of room for further study and categorically does not stop schools offering subjects outside it. Our White Paper made clear that it is 'only one measure of performance and should not be the limit of schools' ambitions for their pupils'."
But critics have complained of unintended consequences, with schools chasing this new target and limiting support for subjects outside the baccalaureate.
Dave Peck, from the Curriculum Foundation, told the meeting there were schools where pupils who had already begun their GCSE courses were being switched into baccalaureate subjects.
There were other schools introducing "twilight classes" to increase the proportion of pupils reaching this benchmark, he said.
This was being driven by the pressure on schools to perform well in league tables, he added, rather than the interests of pupils.
In response to this, an education department spokesman said: "It is simply wrong for anyone to be shipped from one course to another midway through their studies."
But Mr Burnham said: "The danger is, it puts the interests of the school above the interests of the individual."
The NASUWT teachers' union said it had invited responses from its members about the impact of the E Bacc - and among those who had replied, 43% had said their schools "had put in place plans to restrict the degree of choice pupils are able to exercise over subject options".
The teachers' union members reported a reduction in provision of art, music, religious education (RE), citizenship, drama and personal, social and health education (PSHE).
'Freedom of choice'
Peter Hall Jones, also from the Curriculum Foundation, said the English Baccalaureate would "distort" the focus of teaching in many schools.
There have been particular complaints about the absence of religious education from the subjects that count towards the English Baccalaureate.
Jonathan Romain, representing the Accord coalition of faith and humanist groups, said there was a "mantra of freedom choice, but the reality is the narrowing of choice".
Mark Chater, from the Citizenship Foundation, said there was a "troubling contradiction" underlying the English Baccalaureate.
He said there were two things wrong with the new measure: "It's not a baccalaureate and it's not English."
He argued it lacked the breadth to be a genuine baccalaureate - and that prescribing subjects from the centre was against the grain of English education.
It was "Napoleonic", he said, and warned of a "catastrophic" impact for some subjects.
Shadow Schools Minister Kevin Brennan said this was about delivering political results rather than academic benefits - and that it would allow the government to claim a "success story" when English Baccalaureate scores increased.
"Schools will steer resources and children into these subjects, whether it is appropriate for them or not. More pupils will take these subjects."
Last week the government highlighted research that showed pupils who had achieved the English Baccalaureate were much more likely to continue to higher education - and were much less likely to be "not in employment, education or training" or so-called Neets.
The government argues publishing English Baccalaureate figures will encourage a wider availability of these tougher academic subjects to pupils, particularly in poorer areas.
There have been claims some schools have inflated their exam performance by putting pupils into "softer" subjects, which have been of less value to individual pupils when they leave school.