can also reveal that the introduction of a compulsory EBac will mean changes to the government's Progress 8 league table measure, which is due to be introduced in 2016.
After winning the election, prime minister David Cameron stated that his government would implement the Conservative party's manifesto in full. This includes the pledge that all GCSE pupils will be required to study EBac subjects: English, maths, a science, a humanity and a modern foreign language.
Headteachers are concerned that this will limit the subjects their students can take in order to meet the government's new league table measure, Progress 8, which will replace the five A*-C benchmark.
Under the proposals, the Progress 8 measure will track students' performance in eight subjects split into three groups: English and maths, which will be given double weighting; three EBac subjects; and three optional subjects.
But in order to comply with the EBac, students will now be required to take a science, a humanity and a language, potentially squeezing out other subjects.
Fears for flexibility
David Blow, headteacher of the Ashcombe School in Surrey, said the move was likely to cause "very substantial problems" for his students, who would want a greater focus on more creative subjects.
"Progress 8 allowed some flexibility over what subjects students were able to take, but by introducing a compulsory EBac you are effectively taking one option away from them," he said. "It also just adds to the confusion. We will have students taking some subjects on the 9-1 grades, and others on the A*-G, and potentially further complication with a compulsory EBac."
His concerns were shared by Tom Sherrington, headteacher of Highbury Grove School in North London, who said that although options at his school already complied with the EBac, the move would impact on creative subjects.
"We already ask all our students to take a science, humanity and language at GCSE, but we also demand they take an arts subject and there is a real risk that the take-up of the arts will decrease massively," he said.
"It could also hit some schools' performance when it comes to progress, as a student more inclined to study creative subjects would be forced to sit languages or humanities instead."
Under the Tories' manifesto pledge, Ofsted will hold schools to account over whether students are gaining the EBac. And no school will be judged "outstanding" if it "refuses" to offer the full suite of EBac subjects.
The details of how the compulsory EBac will be implemented have yet to be ironed out, but schools minister Nick Gibb is expected to make an announcement in the coming weeks.
A source within the DfE denied that the introduction of the compulsory EBac would lead to significant changes to the Progress 8 measure. "It will undeniably be more prescriptive," he said. "But the discussions are ongoing as to how prescriptive the requirements will be."
He added that university technical colleges and studio schools would be exempt from the EBac.
Impact on less able
John Tomsett, headteacher of Huntington School in York, says any changes to Progress 8 resulting from a compulsory EBac are unlikely to have a big effect on more academic pupils. But it's a different story for the less able.
"We have always made languages compulsory, so the change won't make that much difference as a whole," Mr Tomsett explains. "But we always have a handful of students who are towards the bottom end and this will have the biggest effect on them.
"For those students who benefit from a more nuanced and tailored curriculum, they are the ones who will be hit by this."