Independent schools are more likely to challenge exam results and be successful in their requests for reviews of marking than the vast majority of state schools.
Heads leaders are warning that this means that pupils in state schools, already hit by real terms school funding cuts, are being "doubly disadvantaged".
At both GCSE and A-level, independent schools put in more challenges to exam boards than state schools, according to an analysis of the latest figures from the exams regulator, Ofqual.
Independent schools requested reviews for 8.1 per cent of their GCSE entries – which is more than academies (5.5 per cent) and other comprehensives (5.6 per cent), the 2016 figures show.
And the analysis reveals that 20.9 per cent of GCSE grades challenged by independent schools were changed, compared to 17.3 per cent for comprehensives and 17.5 per cent for academies.