The NAO has recently published research examining the use of rewards and sanctions in government. (http://tinyurl.com/4wkxt4) The document considers how effective these are in driving improvements in public sector delivery. Education is mentioned. The report is not impressed by the use of threshold based sanctions such as the present 30% used with schools.
'“threshold” schemes, which target absolute levels of performance and do not take past performance into account, may not reward Agents who improve the most as result of starting from a lower base.' (Page 27)
The material on unintended consequences of reward and sanction regimes should be required reading for anyone working within the public sector.
Of course the question does arise - if the NAO are saying that threshold schemes aren't a good idea - why isn't the government listening. (As if I didn't know).