In the first test of the system since last summer's A-level crisis, thousands of sixth-formers will sit AS and A2 papers. The estimated entry figures include pupils with unexpectedly poor grades who were left unsatisfied by the Tomlinson inquiry into A-level standards.
The review, ordered by former education secretary Estelle Morris, led to about 10,000 unit upgrades, giving nearly 2,000 pupils improved results in one subject. But many pupils were unaffected by the exercise and have instead decided to take papers again.
OCR, the exam board at the centre of the regrading inquiry, estimates January entries of about 330,000, up from 260,000 in 2001. AQA's entries have reached 600,000, compared to about 430,000 this time last year, and Edexcel reports 250,000 exam entries.
It is too early to say exactly how many of the entries are resits, but the exam boards said the proportion was likely to be higher than last year.
John Dunford, general secretary of the Secondary Heads Association, said:
"The events of last summer caused a loss of confidence among students and parents which will have led to many more resits."
At King Edward VI school, Handsworth, Birmingham, about 60 sixth-formers sit exams this month. Head Elspeth Insch said: "We don't want to encourage resits, but we do feel that if the option is there we have to use it."
Keith Maglennon, head of psychology at Orpington College, Kent, said practically all his students had to be re-entered. "We have well over 100 doing resits because of the results in AQA coursework," he said.
Earlier this year, the Qualifications and Curriculum Authority said students are regarding their first attempt as a mock exam. An analysis last year by OCR of entries in 10 subjects found that nearly 40 per cent of students had resat units. More than 80 per cent gained extra marks.
The second phase of the Tomlinson inquiry said ministers and the QCA needed to urgently simplify the rules governing resits.
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