The performance of GCSE and AS-level pupils will suffer as a result of sitting four or five exams in a day this year, headteachers warn.

This year’s exam season begins on Monday with Key Stage 2 tests for 11-year-olds, followed by GCSEs and AS-levels.

Teachers’ leaders have warned that there is a wider range of subjects and different qualifications with more potential for clashes in timetables than in previous years.

The National Association of Head Teachers (NAHT) has called on the three main exam boards to coordinate their schedules better.

It has already raised its concerns with ministers that the situation is likely to become even more confused, with schools preparing for the introduction of 51 new academic and vocational papers in addition to GCSEs and AS-levels.

The problem is worsened because many pupils, particularly at independent schools, are sitting international GCSEs in addition to conventional ones.

The mother of one 16-year-old boy, who will sit nine exam papers in two days, said she was very angry about the “totally unsatisfactory” scheduling.

Her son, who attends a popular west London independent school, will sit five exams on one day next week, including three international GCSE papers, then another four the following day.

She said: “My son is particularly nervous about these two days. He feels he is particularly under pressure on this.”

Mick Brookes, NAHT general secretary, said exams would always be stressful, but the timetabling was making it worse for both pupils and teachers.

“Nine exams in two days will inevitably cause distress, and must have an effect on the pupil’s outcomes,” he said.

Teachers’ unions will meet next week with the Joint Council for Qualifications (JCQ) to discuss a draft timetable for 2009.

The JCQ has said it aims to minimise clashes, but some are inevitable as it tries to fit about 3,000 exam papers into 30 days. It must fit those papers around religious holidays, Bank holidays and weekends.

Chris Woodhead, former chief schools inspector, blamed ministers for encouraging the wider range of GCSE and A-level subjects.

He told the BBC that exam boards could better co-ordinate the exam timetables.

He added: “But I also think that ministers are responsible because they've encouraged more and more subjects to be offered at both GCSE and A-level and, of course, the more subjects that are offered, the more likely it is that there are going to be clashes between different subjects and different exam boards.”

Emma Horstrup, assistant head at Rainham Mark Grammar School in Kent, said: “We are getting a bunching of exams in one day. Some students are sitting three exams in one day - this is causing great stress and may be affecting their results.”

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