School leaders heckled the education secretary Justine Greening today over claims that grammar schools help close the attainment gap for disadvantaged pupils.

Heads shouted “rubbish” and audibly groaned as Greening took questions about her plan to expand selection during an otherwise ordered and polite session at the Association of School and College Leaders (ASCL) annual conference in Birmingham this morning.

The government is under pressure to justify its plans to open the first new grammar schools in England in decades, in the face of a large amount of evidence that shows that they have a negative effect on social mobility.

Paul Cornish, the principal of Newton Abbot College in Devon, was the first to quiz Greening, and asked about how she could talk of the need for the use of evidence in education policy when her plan “flies in the face of evidence” about social mobility for disadvantaged pupils.

But Greening remained defiant, claiming that poorer pupils do better in selective schools

“We have to recognise that actually for grammars, in terms of the disadvantaged children that they have, they really do help them close the attainment gap,” she claimed, prompting shouting from delegates.

Members of the union also reacted with anger to Greening’s claim that parents “also want choice for their children” and that grammar schools are “often very over-subscribed”.

The government wants to overturn the existing ban on new grammar schools, and will spend £50 million a year helping existing selective schools to expand.