Theresa May has insisted grammar schools will have to show they are "genuinely reaching out" to pupils from poorer backgrounds.
The Prime Minister also said the Government is seeking to find new ways to identify "struggling" children rather than rely on information detailing who receives free school meals.
Mrs May added the education system is "not going back to the 1950s", as she defended her proposed reforms to allow new selective schools.
She is expected to face opposition, including from Tory MPs, when she tries to get her plans through Parliament.
Speaking on The Andrew Marr Show on BBC One, Mrs May said: "We are not going back to that system of binary education. We're not going back to the 1950s."
She added: "There will be different types of schools providing education. What I've always said throughout my political career... is we want the education that is right for every child."
Mrs May went on: "We'll be saying to grammar schools and people who want to set up new selective schools, actually, if you're doing that, we will want you to show that you're genuinely reaching out across society in giving those opportunities to young people and also that you're ensuring..."
Mr Marr intervened to ask Mrs May if she was suggesting targets for those children receiving free school meals - a nod to those from poorer backgrounds - and developing new feeder schools.
She replied: "It could mean a variety of things, Andrew, that's the point.
"We're consulting at the moment as to the best approach in this but it will be about ensuring that when selective schools are expanding or being set up that they are reaching out, that they are ensuring the quality of education throughout the system."
Mrs May added about the consultation: "It's also about how we identify those children - free school meals has been something that's always been used as a measure in education.
"Way back when I was chairing an education committee more than a quarter of a century ago we were talking about other measures and trying to find other measures, so we're looking at how we identify those people perhaps not captured by that but who are struggling."