Schools will struggle to cope with the unprecedented pressure of sorting out the "big mess" created by the Westminster government's simultaneous introduction of two major new sets of exams, teachers' leaders in England have warned.
Last week's decision to delay the implementation of A-level reforms by a year means secondaries will have to teach the new post-16 exams from 2015, the same year that English Baccalaureate Certificates (EBCs) will replace GCSEs in core subjects.
The long-expected A-level plan was finally revealed by education secretary Michael Gove last week and includes a return to single sets of exams at the end of linear two-year courses. AS levels will revert to being standalone qualifications.
Universities will be consulted on new A-level content through a new advisory group, but Mr Gove has had to retreat from his original plan that they should "take ownership" of the exams and formally endorse them.
The education secretary has also had to back down from his aim of introducing the A levels for teaching from September 2014, a goal that exam boards had already concluded was no longer achievable, even for "priority" subjects. The new timetable buys them an extra year but will increase the strain on schools.
Russell Hobby, general secretary of the NAHT heads' union, called for ministers to rethink the timetable. "Don't do it all at once," he said. "I think it would be a huge challenge for schools."
Mary Bousted, general secretary of the ATL teaching union, said: "When you combine reformed A levels with EBCs, the delay of the secondary national curriculum and the massive delay in the primary curriculum, really what you are getting is one big mess."
Mr Gove said he expected "the first new A levels in facilitating subjects" (maths, further maths, English literature, physics, biology, chemistry, geography, history, and modern and classical languages) to be ready for teaching in September 2015, at the same time as new EBCs in English, maths and the sciences.
The announcement came after a new league table measure for A levels was published earlier this week (see panel). It also followed a letter signed by nearly 100 groups from education, the arts, design and industry, which called for Prime Minister David Cameron to delay the pace of EBC reform.
NUT general secretary Christine Blower said that there was an "unmanageable level of change that could lead to a collapse of the system".
Exam boards will welcome the extra time to develop new A levels but privately warn that they still face a "very tight schedule" to get them ready on time.
The University of Cambridge said the removal of the AS level from A levels would "jeopardise over a decade's progress towards fairer access".
Mr Gove claimed support from higher education, saying that "many leading universities are concerned about current A levels". But the Russell Group of elite universities, which will set up the new group on A-level content, has said that, although it has some concerns, existing A levels are "broadly fit for purpose".
Mr Gove said: "These changes will enhance the reputation of A levels, better prepare more students for higher education and ensure that competition for university places is fairer."
Tables published by the government last week included a new column showing the percentage of pupils at each sixth form who gained at least two As and a B in A levels in "facilitating subjects".
These are subjects identified by elite Russell Group universities as their most commonly required A levels, including maths, further maths, English literature, physics, chemistry, biology, geography, history, and modern and classical languages.
School leaders fear that a new A-level league table measure could lead to students being pressurised into taking unsuitable courses.
The tables for the first time showed separate results for boys and girls at each school.