Lynda Warrington, president of the Girls' Schools Association, said that heads were concerned about social pressure on their pupils.
"Children have to consider things like smoking and alcohol at an earlier age," she will tell the GSA's conference in Newport, Wales, on Monday. "They don't have time to be children any more.
"Parents have a very difficult task in terms of how they give guidelines to young teenage daughters because of the commercial and media pressures.
"Things appear on TV encourage children to believe they are adults and that they can cope."
Mrs Warrington, head of Bradford girls' grammar, said she would e calling on schools to work with parents to keep social pressures, for example, to have under-age sex and to misuse alcohol, at bay.
"We are concerned as heads about how these issues impact on the girls in our care," she said."Issues which young girls seem to think are important in their lives."
The association, which represents 200 schools, will also look to consolidate their domination of the examination league tables by focusing on the importance of good leadership in schools.
Dame Pat Collarbone, director of the leadership centre at London University's Institute of Education, will speak at the conference, as will Julia Cleverdon, chief executive of Business in the Community and Sarah Stevens, policy adviser at the General Teaching Council.
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