from NFER published July 2014 (added by Janet Downs directly from the link provided by Peter Martin, her emphasis):
1 'Attainment progress between Key Stage 2 and Key Stage 4 outcomes, such as capped point score and percentage achieving 5+ A*-C grades including English and maths, is higher after two years in sponsored academies compared to similar non-academy schools.
2 There was no significant difference in attainment progress after two years between converter academies and similar non-academy schools, suggesting the school performance benefits are limited, at least in the short term.
3 Attainment progress in sponsored academies compared to similar non-academies is not significantly different over time when the outcome is measured as GCSE points, excluding equivalent qualifications such as BTECs. This suggests that sponsored academies either use more equivalent qualifications, or that their pupils do better in them. The same was found for converter academies, though to a lesser extent than in sponsored academies.
And from the full report
(added by Janet Downs, her emphasis):
'Analysis of 2013 exam results appears to show more progress amongst converter academies than all non-academy schools, especially among the very first converters, that became academies in 2009/10. These schools were all rated ‘outstanding’ by Ofsted at the time, so greater progress made in 2013 might be better explained by pre-existing differences rather than the impact of academy status
'A more robust longitudinal analysis shows no significant difference in attainment progress after two years between converter academies and similar non-academy schools
, suggesting the school performance benefits are limited, at least in the short term. This could be interpreted as mean reversion counteracting a positive academy impact, though mean reversion has been partially addressed by excluding non-academy schools from the analysis that are not a good comparison with academy schools. A longer time frame may be needed to fully assess the relative performance of converter academies, but the data so far suggests academy status has made no difference to the progress made in converter academies, compared to similar non-academy schools over the same time period