Heads are angry that their schools are to be retrospectively judged on the Government's new "English Baccalaureate" in GCSE league tables being published next week.
Ministers have moved the goalposts by calculating the new Bac on exams that were taken last summer, at least two months before the measure was announced, according to school leaders.
The English Bac will be awarded to pupils who achieve at least a grade C in English, maths, a humanity, a language and either two separate sciences or the combined double science exam. IGCSE grades will also count towards the Bac and will be included in official tables for the first time.
The Government is concerned that existing tables can encourage schools to offer pupils "soft" subjects instead of a broader academic education. But heads have complained it is unfair to introduce the new measure without giving schools adequate warning.
They fear it will leave vulnerable schools in disadvantaged areas open to unfair criticism and lead to other subjects being neglected as schools chase the new benchmark.
"There is a real risk of narrowing the curriculum for the most able as well as those who thrive on practical rather than classroom-based learning," Brian Lightman, Association of School and College Leaders general secretary, writes in The TES this week.
There has been controversy over the content of the Bac since education secretary Michael Gove announced it in September last year.
Only history and geography qualify as humanities, prompting complaints about the omission of religious education. Mr Gove told MPs last month he accepted there was a "debate" and that some thought the definition of humanities was "too narrow". "I don't want to pre-judge the way in which aspects of that debate on the national curriculum will go," he said.
Mr Lightman said the Government should have waited for the curriculum review before introducing the Bac. "You don't tweak the curriculum by reviewing the performance tables," he said. "That is the tail wagging the dog."
Many heads also question the sense of including Biblical Hebrew and ancient Latin and Greek in the Bac's language section, he said.
Ian Johnson, head of Marlowe Academy in Ramsgate, said his school will change its curriculum in line with the new Bac, but that it would take two years to introduce.
"It's impossible to change for the class of 2012, because they've already made their choices," he said. In the meantime, "there will be a shedload of schools with a big fat zero in that column (on the league tables)".
Joan McVittie, head of Woodside High School in north London and ASCL vice-president, said: "We have already started having discussions about changing the curriculum.
"You need to have a curriculum that suits your children, but we certainly don't want to be pushed back down the league tables - we've been a National Challenge school and we don't want to go back there again."
The Department for Education has said it will review the Bac's "precise definition" for next year's tables but that it "wouldn't expect to remove any of the qualifications" already included. The existing 5A*-C GCSE including English and maths measure will remain and has Mr Gove's backing as the "most important" measure of school performance.
Insight, page 31.