Students are re-sitting their GCSE exams up to seven times so their school can improve its ranking on national league tables, an investigation has revealed.
More schools are adopting the unorthodox practice as they go to extreme lengths to "play the system", the Department for Education found. Pupils are sitting exams for the same subject with multiple exam boards to increase their chances of scoring top marks.
Last November, it emerged teenagers at hundreds of secondary schools were being entered for both the GCSE and the IGCSE, the exam favoured by private schools, to boost results. The DfE's research found students taking English in the two exam versions increased from 300 to 4,000 in 12 months.
During the same period, the number of students taking maths had increased tenfold, 400 students re-sat exams up to seven times.
The DfE said it was "increasingly concerned", saying the practice was harmful to pupils.
"We are increasingly concerned about this. The evidence shows that entering exams early, and then re-sitting, or sitting another exam in the same subject, is not good for pupils.
"The changes we have made to GCSEs and reforms to the accountability system will help address this. We are considering further action to discourage this practice."
The exam system has come under heavy criticism in the past year, with Ofqual's chief Glenys Stacey being held to account by the Commons' education select committee after the GCSE markings scandal.
The exam watchdog recently sent a letter to schools warning there would be a "small drop" in GCSE science grades following tougher exams.