They have written to Sir Ron Dearing, who heads the Government's advisory body on exams, arguing that the marking system is 'the most imprecise ever used in a public examination'.
In the letter, the National Association for the Teaching of English asks him to intervene to sort out the muddle over how grades are awarded.
It points out that, though the syllabuses and exam papers were based on the national curriculum 10-level scale, under which pupils were graded from 10 (top) to one, the Government has now decided to revert to the former O-level grades A to G.
Teachers will mark coursework according to the levels, which will have to be matched to grades by exam boards, but schools are worried that the crucial grades C and D, the O-level boundary, have no exact equivalent on the 10-level scale.
The whole nature of the exam is different this year because coursework will account for only 40 per cent of the marks, compared with 100 per cent in the case of two-thirds of last year's candidates. For the first time, teachers must choose between papers of varying difficulty for their pupils.
English teachers, whose protests last year led to a test boycott by three teacher unions, say pupils do not have enough chance to show what they can do because of the reduction in coursework.
Anne Barnes, the association's general secretary, said: 'A lot of injustices are going to be done. We are sacrificing a whole cohort of 16-year-olds to a completely impossible exam. These exams should be seen by colleges and employers as a pilot.'
Differences between exam boards and highly complicated mark schemes are compounding the problems, the letter says.
It adds that some boards have been specific about the sort of work coursework should include, others vague.
Some have separate papers for reading and writing. Others give a mark for each on both papers.
Teachers argue that deciding which of the different papers a child should sit is particularly hard in English because it is difficult to predict which passages a child will find easier. Personal interest and experience play a key role.
As a crude example, Mrs Barnes points to papers set by one of the boards in which the higher level involves a description of sailing a yacht and the lower a piece about whether or not to put a grandparent in a home. A Bengali child of lower ability might find the first easier to grasp than the second.
A spokesman for the School Curriculum and Assessment Authority, which Sir Ron chairs, said: 'The introduction of new exam syllabuses always raises new issues. We are in regular contact with all the GCSE boards on this matter and recently hosted a joint conference to identify any remaining difficulties. The boards have been offered further meetings, as necessary.'
George Turnbull, of the Southern Examining Group, one of the five GCSE boards, said that teachers had been worried about marking when GCSE coursework was first introduced, but their fears had proved unfounded.
He added: 'We have so much experience at adapting very quickly to changes during the last few years that we feel we can take this in our stride.'Reuse content