THE National Union of Teachers is threatening a High Court challenge against the government’s “back door” expansion of grammar schools, its leader said yesterday.

New academically selective schools have been banned since 1998 — but some academies are now opening “grammar streams” where kids are hived off via ability tests.

The NUT has written to Education Secretary Justine Greening arguing that a clause in the government’s recent education green paper encouraging academy chains to “establish a single centre in which to educate their ‘most able’ pupils” is unlawful.

The green paper says the practice is “currently permissable” as schools are allowed to set up ability streams within schools after they have been admitted.

But the union says this goes much further than traditional streams and segregates kids in a far less flexible fashion, with some schools testing pupils prior to admission.

NUT solicitors have sent letters to individual schools asking them to explain the situation.

General secretary Kevin Courtney said: “There are schools that are very publicly advertising they have a grammar stream.”

The union’s senior solicitor Clive Romain said schools could be “subject to court action” if they were found to be in breach of admissions procedures.

The government could also find itself in the dock on the basis that schools were acting on unlawful guidance from Whitehall.

A further possible challenge would involve reporting individual institutions to the Schools Adjudicator.

“When parents send their children to what they understand is a comprehensive school and they then learn that in fact there is a particular grammar stream, which is created from a certain group of children who then, throughout their whole school career, are in that privileged grammar stream … they can be quite understandably concerned about that,” Mr Romain said.

Mr Courtney said his organisation is taking action now as it fears the government is trying to expand grammar schools via the back door to avoid legislating for new ones in Parliament.

“We know the government has no manifesto commitment for grammar schools,” he said.

“We think they are aware it would be a difficult parliamentary passage.”

Ms Greening suggested new grammar schools would benefit kids “from all backgrounds” on Thursday.

The government’s own statistics showed fewer than one in 10 grammar school pupils came from the most disadvantaged backgrounds, compared with over one in six at non-selective schools.

But Mr Courtney said a grammar school “open to all” was an “oxymoron,” adding: “Schools that are open to all sound a lot like comprehensives to me.”

He said any attempt to guarantee grammar school places for poorer kids would be “saying you are the deserving poor … and you are the undeserving poor.”

An emergency motion being heard at the NUT’s conference in Cardiff today will assert that “proposals to allow more grammar schools and increased selection are wilfully evidence-free.”

It says new grammar schools will “exacerbate inequality” and that “no 11-plus test can be ‘tutor-proof’.”

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 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.”