I know quite a few posters here have expressed concerns about their DC's schools entering students early for GCSE exams, and it turns out rightly so. Students entered early for Maths or English GCSEs do worse than those entered at the end of Y11 even when you take resits into account.
(note: early entry is not the same as taking modules in Y10, it is completing the entire GCSE before the end of Y11)
The DFE has just published a report analysing the statistics and found 'The statistical evidence so far indicates that for lower achieving pupils, there is little discernable benefit in early entry other than increasing opportunities to retake before the end of KS4 which still does not improve the overall comparative result. For pupils who achieve level 4 or above at KS2 (and would therefore be expected to achieve grade A*-C at GCSE) the average final grade is lower for early entrants. Higher attaining pupils are therefore being disadvantaged by entering early and not achieving their full potential.'