Some schools pointed out that the A2 is yet another unknown quantity as it is marked to a tougher standard than the AS exam. Other headteachers said they had been reassured by good results from modules taken in January.
As the exam season begins in earnest, an Association of Teachers and Lecturers' survey found students are suffering from panic, headaches and depression because of the pressure.
Gwen Evans, deputy general secretary, said: "The constant reports of timetable clashes, students having nine hours of exams and exam-board errors are disheartening for pupils and teachers."
Key skills papers taken last week were beset by problems. The exam watchdog, the Qualifications and Curriculum Authority, admitted to setting an unanswerable question in the communications papers. Some schools received question papers with pages in the wrong order. Exam board Edexcel blamed a printing error.
Sue Kirkham, headteacher at Walton high school, Stafford, said: "Students had to cross reference to three pages faxed through at the last minute. It is just not good enough." Edexcel is also investigating a complaint that key skills information technology papers failed to arrive at a Nottingham college.