Ofsted inspectors warn this could leave them vulnerable to sexual exploitation.
Too few teachers have the expertise to discuss delicate issues such as sexuality and domestic violence, they say.
The warning comes after teaching unions raised concerns about the effects of a sexualised culture on pupils.
At unions' conferences over the Easter holidays, teachers shared their concerns about the negative impact pornography and pressure to have "the perfect body" was having on their pupils and called for better training to help teachers to deal with such issues.
In a report examining personal, social, health and economic (PSHE) education, Ofsted found it was good or outstanding in 60% of schools, but requiring improvement or inadequate in 40%.
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 there was "less emphasis on sexual consent and the influence of pornography."
"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 says.
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.
"In 20% of schools, staff had received little or no training to teach PSHE education. Teaching was not good in any of these schools."
Ofsted's report found the development of pupils' personal and social skills was good or outstanding in 42 of the 50 schools inspected, but in the weaker schools "casual use of homophobic and disablist language was commonplace".
"In just under half of the schools, pupils learnt how to keep themselves safe in a variety of situations but not all had practised negotiating risky situations or applied security settings to social-networking sites," the report says.
In March, Education Minister Liz Truss announced  PSHE would remain a non-compulsory subject, saying it should be down to schools and teachers to decide on the topics covered in lessons. Sex and relationships education is, however, compulsory.
PSHE Association chief executive Joe Hayman said the report  painted a realistic picture of provision across the country.
"The reality is that while there is outstanding practice in many schools, far too many teachers go into PSHE lessons ill-equipped to deal with the extremely important and challenging issues the subject covers.
"It is obvious from the report that teachers need more help than they are currently getting and as a result many pupils do not get the high quality PSHE education they deserve."
A Department for Education spokesman said: "The quality of PSHE teaching is not good enough.
"Our curriculum reforms have given teachers the freedom to tailor their teaching so it meets the needs of their pupils.
"We are funding the PSHE Association to work with schools to develop curricula and improve the way it is taught.
"The best people to fix this problem are teachers on the ground - not politicians in Westminster."
The findings of the Ofsted report were based on inspections of 24 primary, 24 secondary and two special schools between January and July 2012.
More than a third of schools in England are failing to provide pupils with age-appropriate sex-and-relationships education, the schools watchdog says.