Ofsted has warned that a lack of sex education in school is leaving children vulnerable to sexual exploitation.
It says more than a third of schools in England are failing to provide pupils with appropriate sex-and-relationships education because teachers lack the expertise.
In a report examining personal, social, health and economic (PSHE) education, Ofsted found it was good or outstanding in 60 per cent of schools, but requiring improvement or inadequate in 40 per cent.
In primary schools, the report says, too much emphasis is placed on friendships and relationships when teaching sex-and-relationships education and this can leave pupils ill-prepared for the physical and emotional changes of puberty.
And in secondary schools, too much emphasis is placed on the 'mechanics' of reproduction rather than the importance of healthy sexual relationships.
The report says: "A lack of high-quality, age-appropriate sex-and-relationships education in more than a third of schools is a concern as it may leave children and young people vulnerable to inappropriate sexual behaviours and sexual exploitation.
"This is because they have not been taught the appropriate language or developed the confidence to describe unwanted behaviours or know where to go to for help."
Ofsted found that most secondary schools cover topics such as puberty, reproduction, sexually transmitted infections, contraception, abortion and pregnancy in PSHE lessons, but added: "The failure to include discussion of pornography is concerning as research shows that children as young as nine are increasingly accessing pornographic internet sites, and ChildLine counsellors have confirmed an increase to more than 50 calls a month from teenagers upset by pornography."
The report calls for better training for those teaching PSHE, particularly over 'sensitive issues'.
"Too many teachers lacked expertise in teaching sensitive and controversial issues, which resulted in some topics such as sexuality, mental health and domestic violence being omitted from the curriculum," it says.
"This was because subject-specific training and support were too often inadequate."