Parents in England are to be given the right to request schools provide childcare for the full working day during term time and in the holidays.
Education Secretary Nicky Morgan said schools must take “reasonable steps” to ensure this is provided.
Ms Morgan said her party must champion the interests of children and parents.
Head teachers warned that the idea would have to be handled “extremely carefully to ensure it is not just a populist gimmick”.
Speaking at the Conservative party conference, Ms Morgan said: “We’re going to give more working parents something the best schools already do.”
“We will be giving families in thousands of schools a ‘right to request’ their school provides childcare for a full working day, before and after school and during the school holidays.
“If enough parents call for childcare at their local school, we will expect the school to take reasonable steps to accommodate it, in a way that works for them.
“Because we want working parents to have the confidence their child is in a happy and safe environment.”
But Russell Hobby, general secretary of the National Association of Head Teachers, warned that while wraparound childcare in schools was generally a good idea, there were many reasons – such as budget cuts or recruitment issues – why some schools were not in a position to offer it.
He added: “Extending provision beyond 38 weeks, providing care outside of term time, can also prove very difficult for schools because of staffing and a lack of private provision.
“Parents can ask but the government must guarantee that a school’s decision is respected. Otherwise, it is merely going to provoke conflict between schools and their communities and would undermine the decision-making of head teachers.
“This needs to be handled extremely carefully to ensure it is not just a populist gimmick.”
Ms Morgan also used her speech to tell the conference that the Tories had “raised the bar on standards in schools with a rigour revolution”, for example by ending grade inflation and introducing a “tough new national curriculum”.
She said politicians and bureaucrats had been taken out of the classrooms, with 3,000 head teachers in good and outstanding schools trusted with the freedom to run those schools.
A thousand failing schools had been transformed under the leadership of strong sponsors, she said, and more than 300 free schools had been set up by parents, teachers and community groups.
Ms Morgan also told the party-faithful the Conservatives had speeded up the adoption process to help young people find a loving home and provided “joined-up support” for children with special educational needs and their parents.
Ms Morgan told delegates that what united every Conservative was a belief in meritocracy.
“That commitment to meritocracy means nothing if we don’t give every child the chance to succeed,” she said.