Members of the NASUWT, meeting at its annual conference in Manchester, heard concerns that the new system will result in added pressure being put on pupils.
The reforms mean the traditional A*-G grades have been axed in English and maths, and will be replaced by the 9-1 grades - with 9 the highest result.
The changes will be brought in for all subjects in future years.
Supporters have argued the move is necessary to allow more differentiation between students.
But there have been concerns raised over a lack of clarity about the new system.
NASUWT general secretary Chris Keates said: "Maintaining high standards of education is critical and it is important that the qualifications system is kept under review to ensure it is meeting the needs of pupils, employers and the further education sector.
"However, these changes have been driven by political imperative, rather than the needs of young people.
"The Government has consistently sought to portray GCSEs as 'broken' and 'dumbed down' qualifications in order to push through its vision of an elitist, narrowly focused curriculum and qualifications system which risks failing to meet the needs of the majority of young people.
"The changes to exam grading have created huge uncertainty for pupils, teachers, parents and employers which will be difficult for schools to manage.
"Schools already buckling from excessive workload are now facing even more bureaucratic reform and young people, already experiencing rising rates of anxiety and mental ill health, will face even greater pressure to perform."
Additional reporting by the Press Association.