Schools are being accused of "hot-housing" pupils after figures released yesterday revealed a leap in the numbers of students who are being selected to take their GCSEs early.
Examination boards have reported substantial increases in the number of pupils who took their exams early in November, allowing a resit for any failures - a tactic that is being encouraged increasingly across the country by heads desperate to rise in the league tables.
Some pupils have taken the same GCSE exam up to three times in the space of a year, the Guardian has learned.
Teachers' leaders have blamed the government's pressure on headteachers to improve results and move up the school league tables for the increased focus on exams that was putting children under stress and detracting from the depth of their learning. But some headteachers have defended the practice, saying it gives students an opportunity to do their best.
The league tables, which are published today, rank every secondary in England according to its GCSE and A-level results. Schools' reputations rise and fall with the ranking.
Pupils who sat their exams in November - six months before the exam season traditionally begins - will also get their grades today.
The exam board, Edexcel, said it had seen a 67% rise in the number of pupils taking a whole GCSE early, while the number of pupils re-taking modules to improve their scores had nearly doubled, to 800,000 entries. The practice of early entry is rising fastest in English and maths GCSEs. Last year the government announced that any schools where fewer than 30% of pupils achieved at least five good GCSEs - including in English and maths - faced closure unless there were improvements within three years.
Today's league tables will reveal the schools that have moved in and out of the threatened category, but separate figures released yesterday by government statisticians confirmed that 440 schools were failing to meet the 30% target.
About 200 schools are believed to have left the category of threatened schools placed in the "national challenge" group in the past year, but pressure to improve will intensify on those remaining.
Alan Smithers, professor of education at Buckingham University, said: "League tables have got all out of proportion and schools will now do all they can to improve their place. Early entry is one way they are doing it. Other ways include focusing on the pupils on the C-D border. We're in danger of producing a set of statistics that no longer accurately reflect pupils' progression but the work the schools can do to improve their scores."
Perry Beeches school in Birmingham has adopted early entry for its entire GCSE group, with nearly all of its pupils sitting their English exams a year early and maths six months early. The pupils were allowed to retake, or sit a harder paper to improve their result. The school is believed to have made the biggest improvement of any school in the country in its GCSE results, increasing the number of students getting five good GCSEs, including in English and maths, from 21% in 2007 to 51% in 2008.
Liam Nolan, the school's head, said: "My students say they are really glad of early entry because they say it spreads the load of pressure across the year. We are an exam factory, I have no issue with that. However, this is about the students. They've got to pass these exams to give themselves every opportunity in life."
John Bangs, head of education at the National Union of Teachers, said: "Retakes are bad for kids. There's a poisonous combination of parental pressure and league-table pressure to get as high a score as possible. There are kids who are ready to take their exams early but it should be on the basis of their readiness rather than part of a process to get the results.
"What it does is focus on the examination results rather than the use of English and maths across the curriculum - it's for the sake of the exam itself ... examination result-driven decisions to hot-house kids into taking an exam early are damaging."
Steve Munby, chief executive of the National College for School Leadership, said: "If you have a system of take-when-ready assessment all that schools are doing is putting pupils in to take the exams when they are ready. The negative argument is that they could do even better if they waited and did it in depth."
A DCSF spokesperson said: "Teachers may encourage exceptional candidates to take exams early, if this is something that both the students and parents are comfortable with. We are not aware of widespread issues with students being unduly pushed to take GCSEs early."