Schools are rushing in "dramatic" changes to their curriculums that will cut the time devoted to subjects not recognised in the English Baccalaureate, The TES has learned.
Subjects such as RE and music have already been hit as schools attempt to move pupils on to courses that will count towards the controversial new league table measure.
Heads are even prepared to break their statutory duties to teach RE as they switch resources to other qualifications, the National Association of Teachers of Religious Education (NATRE) has warned.
In a survey by the National Association of Music Teachers (Name), 60 per cent of respondents said their departments had already been adversely affected by the EBac. Music teachers in 57 out of 95 schools said their schools plan to reduce opportunities to study music from this September.
The EBac is awarded to pupils achieving an A*-C in GCSEs or IGCSEs in English, maths, two sciences, a foreign language and geography or history.
The omission of RE from the list of approved humanities and wider arts subjects has prompted an angry response from subject associations, which fear they will be sidelined. Fears have also been raised that teachers of those subjects could face redundancy.
Rosemary Rivett, executive officer at NATRE, said: "We have already heard from teachers that something quite dramatic has happened in their schools.
"Schools are reacting very quickly to the EBac and are realigning their curriculums.
"We have been contacted by some schools that are willing to no longer provide RE in Year 11, even though it is a statutory requirement."
Sarah Kekus, chair of Name, said: "Excluding the arts from the English Baccalaureate makes them invisible in school, not only leading to cuts in provision, but also reducing opportunities for young people to gain recognition for what they excel in."
The TES has also been contacted by teachers who said students are being asked to alter their subjects in order to make them eligible for the EBac.
A teacher at a school in East Yorkshire said pupils who have already started their GCSE courses had been asked to switch subjects.
She told The TES: "The school where I teach has already moved some Year 10 pupils ... on to geography if they don't already do it, and other pupils have been invited to consider changing options, decision still pending."
Students who pass their exams early are also being encouraged to take on extra GCSEs in EBac subjects. "Most of our Year 11 pupils sat maths and English GCSEs in November - work is in hand to move those who achieved C or better on to other subjects," she added.
Adrian Prandle, policy adviser at ATL, said: "Just because something is right for the league tables doesn't mean it is right for the pupils.
"This is a real distraction for students, and it's definitely tough for the people trying to teach them."
A spokeswoman for the Department for Education said: "The English Baccalaureate will be only one measure of performance and should not be the limit of schools' ambitions for their pupils.
"Schools will retain the freedom to innovate and offer the GCSEs, IGCSEs and other qualifications which best meet the needs of their pupils."