Sex and relationships education (SRE) is often taught by poorly trained, embarrassed teachers, experts have said.
A review of studies carried out mainly in the UK found that sex and relationships education (SRE) is often "out of touch with many young people's lives".
Researchers found that young people dislike having their own teachers deliver sex education, and said that specialist teachers should be brought in to conduct the classes.
Writing in the journal BMJ Open, they said: "Unless we get the delivery right, young people will continue to disengage from SRE and opportunities for safeguarding young people and improving their sexual health will be reduced."
The review was made up of 55 publications, mainly from the UK, but also from countries including the US, New Zealand, Canada and Sweden.
Focusing on the views of young people, it found SRE often portrayed sex in a negative way and that there was too much focus on abstinence.
Mixed-sex classes were sometimes disrupted by young men, trying to hide their anxieties.
Young people also criticised the overly scientific approach to sex, which ignored pleasure and desire. Men were portrayed as predatory, they said, and there was little or no discussion of gay, bisexual, or transgender sex.
Many young people also felt that SRE should be taught at an earlier age, the review found.
A spokesman for the sexual health charity Brook, which delivers 10 per cent of sex education in schools, said that the findings were not surprising.
“In order to develop healthy relationships, young people should be taught about consent, pleasure, respect and kindness,” he said.
"Brook also believes that relationships and sex education should be delivered within a wider context so that sex and relationships are linked to other issues such as alcohol and drugs."