Jeremy Corbyn’s supposed basis for abolishing SATs is that the new policy would improve fairness and equality in schools. This is bogus, and is demolished in detail by Katharine Birbalsingh, the headmistress of the Michaela Community School:

‘He says they are unfair, but the very opposite is true as they are set by an independent organisation, creating a level playing field for all pupils, regardless of race or privilege. SATs see no colour or class, but only the test performance.

But with teacher assessment, the position was very different. Individual teachers could not help but bring their own expectations of the child to the process, as was clearly proved in a study carried out in the 1990s by the National Union of Teachers (NUT), which revealed how such assessments were riddled with bias.

Mr Corbyn frequently highlights his commitment to equality and diversity, but ironically the biggest losers from his abolition of SATs will be pupils with special educational needs and those from working-class or ethnic minority households.’

Quite what replacement Labour are proposing is unclear – they won’t say what it would be, only that it would be better, which isn’t quite a convincing case. Given that Corbyn’s speech attacked the concept of such testing, it seems unlikely to be a new examination just benchmarked or carried out in a different way.

But if the supposed fairness justification doesn’t stack up, and there appears to be no actual alternative plan beyond ‘do something else’, why exactly would the Opposition leader adopt this new policy?

The answer lies not at the podium during his speech, but in the audience. His speech was delivered at the annual conference of the National Education Union, and it is the teaching unions, not school pupils, whom the policy has been designed for.

What is the purpose of SATs? Obviously the function of the tests is to measure a child’s educational level. But the reason to do so is not to assess a pupil’s ability, achievement or effort – unlike, say, GCSEs. Rather it is to measure the performance of teachers and schools, in order to identify where there are problems.

Despite their efforts to cast themselves as the champions simply of education for its own sake, it remains the case that teaching unions’ primary purpose is to promote and defend the interests of their members. Often that has gone hand in hand with the promotion of educational standards, but sometimes it finds itself in conflict.

The idea of abolishing SATs is a good example of such a clash. The unions have at times opposed the very idea of sub-par teachers even existing (remember the horror at the idea of performance-based rewards?), so a measure which threatens to identify and challenge them to do better is not very popular.

As Birbalsingh and others have pointed out, experiments with teacher assessment – not judging performance on a balanced, benchmarked and independent basis – not only brings further bias into the system but lowers standards, by removing a mechanism to demand improvement. In Scotland and Wales, SATs were removed after devolution only to be reintroduced once the mistake became clear. But that practical evidence – all gleaned at the expense of pupils whose education suffered as a result – does not appear to have deterred Labour for a moment from simply repeating the error.

In short, Corbyn’s idea prioritises the demands of teaching unions over the needs of schoolchildren.