Targets damage standards.

'A report by Teach First has highlighted the damage being done to able pupils by school targets. In essence it says that schools focus on borderline C/D pupils and ignore the needs of the A/A* pupil.
Liz Thonemann the author said “In terms of how it impacts on a school’s reputation, it makes more sense to get kids up to a C than get those at B up to an A*.'

In the comprehensives where classes are larger - this general problem is even more exaggerated.