The NUT teaching union is looking at taking legal action to challenge the “grammar streams” operated by some academies, its general secretary has revealed.

Speaking ahead of the NUT’s annual conference in Cardiff, Kevin Courtney said the union had already written letters to several schools that operate a separate, permanent stream for some children based on a test taken before they join in Year 7.

The letters ask the schools for information on their streams. Mr Courtney said the union believed that there were streams currently in existence which were “illegal” and that it was looking to “pick a test case” which it could take action against.

Mr Courtney declined to reveal the schools to which the NUT had written, but he said they numbered less than half a dozen and were all academies.

Clive Romain, the union’s senior solicitor, said that if the schools contacted were unable to explain what they were doing they would be “subject to a court action to stop them doing what they’re doing, which is unlawful”. 

The schools involved all specifically mention the grammar stream in their marketing materials, he added.

Tes reported last December that the union had written a letter to Justine Greening, the education secretary, claiming that attempts to expand academic selection via multi-academy trusts would break the law. 

In September, the government published its grammar school Green Paper Schools that Work for Everyone, which included a proposal to “encourage multi-academy trusts to select within their trust”.

“We will make clear that multi-academy trusts …[can] establish a single centre in which to educate their ‘most able’ pupils,” the paper states.

However, the union said it had been advised that selection within academy trusts in the manner described in the Green Paper - whether via a specific school within a MAT or a “centre of excellence” that is not itself a school, would be unlawful.

Mr Romain said Ms Greening had not responded to the union’s letter.

He said legal action could take the form of a judicial review against the education secretary’s advice, a High Court challenge against a particular school or a challenge to the admissions regulator.

He made a distinction between grammar streams - which permanently separate pupils by ability - from traditional setting and streaming within schools, where pupils can move up and down between sets.

While there are schools which have operated grammar streams for some time, Mr Courtney said the NUT’s decision to consider legal action now was in response to the government’s plans to expand academic selection.

He said any attempt by the government to bring forward legislation allowing new grammars to be created would face a “difficult parliamentary passage”,  partly because it had not been a manifesto promise - meaning the Lords would feel more able to oppose it - and partly because politicians would be tied up with Brexit negotiations.

He said the union was seeking to “close up” non-legislative routes for expanding selection.

The NUT has added a debate to its conference agenda on the government’s plans to expand selective education. Delegates will vote tomorrow afternoon on whether the union should “investigate possible legal routes to challenge the expansion of selective education.”

A Department for Education spokesperson said: “Streaming pupils by ability is, and has always been, allowed at all schools, and helps teachers give every child an appropriately stretching education.

“Multi-academy trusts (MATs) have always been able to pool their resources to deliver these benefits on a larger scale and across different sites within the trust, and we want to see more do this.”

The spokesperson added that selected pupils can “form a cohort that can be educated on a part-time basis at different sites, provided the pupils remain registered to their original school and receive some of their education there”.

But they must have been admitted to the school on a “non-selective basis”.

The spokesperson added: “The consultation does not seek to make any changes to these existing rules around MATs streaming in this way; instead, we are seeking to encourage more MATs to do this.”