The government has been criticised over its decision to use the English baccalaureate in this year's GCSE performance table.
To meet the criteria for this additional qualification, pupils must gain five A* to C-grade GCSEs in core subjects including English, maths and a modern language.
Christine Blower, general secretary of the National Union of Teachers, said the government should work closely with educators to come up with a suitable testing and accountability system that works in the best interests of pupils and parents.
"Teachers and head teachers will be dismayed that not only has the government once again changed the goalposts for school accountability but also made it retrospective. You can't have schools judged against criteria that were not previously in place," she commented.
"This will significantly disadvantage some schools, as they will not have been geared up to doing, for instance, a modern language."
She added that teachers deserve more than this from the government.