Education Secretary Justine Greening has been heckled by headteachers after claiming grammar schools help to close the attainment gap between well-off and poorer children.
Some members of the Association of School and College Leaders (ASCL) shouted "rubbish" at Ms Greening during the organisation's annual conference as she attempted to defend the Government's plans for more grammars.
After giving a keynote speech at the conference in Birmingham, Ms Greening was asked by a headteacher from Devon why Government policy "flies in the face" of evidence on grammars' impact on social mobility.
Ms Greening told delegates: "First of all, in terms of grammars but also the investment that we announced for more schools beyond 2020, I think it is important that we're planning ahead.
"We know we've got a demographic bulge that's already been in our primary system and that really is flowing into the secondary system as well.
"So we need to have an eye on that and, of course, as a Government we've brought forward the Schools that Work for Everyone consultation to really say 'What is it going to take to really drive social mobility?'."
Dozens of ASCL members jeered as the Education Secretary added: "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.
"And at the same time we should recognise that parents also want choice for their children and that those schools are often very over-subscribed."
Ms Greening went on: "I think we have to respond to that and the consultation wasn't really just about selection it was also about how other parts of the eduction system can also play a role in strengthening our overall school system, whether it was selection, whether it was universities or indeed independent schools and faith schools.
"So we want to make sure that we take on board all of the comments.
"But we also will come forward with what I hope will be a very strong package that doesn't just look at how selection has performed in the past, but very much looks forward to what a sensible approach on selection - which we do have in our system - should be in the future."
The ASCL has warned the Government against creating more grammar schools.
In a speech to the conference, ASCL interim general secretary Malcolm Trobe said: "There is no evidence to support the Prime Minister's current conviction that doing so will improve standards and social mobility.
"Our members leading grammar schools do a fine job with the young people in their schools but although the creation of more selective schools may improve social mobility for a very small number of disadvantaged children, it will not for the vast majority.
"We need solutions which help the many, not just the few. We need as many people as possible to have the high-order knowledge and skills that will enable us, as a nation, to compete on a world stage."
Prime Minister Theresa May has argued that grammars can help the life chances of poor pupils and that the current system sees ''selection by stealth'' based on parents' wealth and ability to buy houses near the best schools.
New Ofsted chief inspector Amanda Spielman, who was also due to address the conference, acknowledged that the issue of grammar schools was problematic for many in the education system.
"It is clearly a very difficult, emotional one for many people," she told BBC Radio 4's World At One.
"We are talking about very small numbers of schools, a tiny proportion of the system, but something that sends a very important message to a lot of people.
"People are very focused on social mobility prospects for disadvantaged children and it is clear that not many disadvantaged children get to go to grammar school although they do do very well when they get there."