Schools should give lessons on pornography to teach pupils that it shows a "distorted image" of sex and relationships, according to new guidance.

Youngsters need to learn that pornography does not reflect real life, often showing "'perfect' bodies and exaggerated sexual prowess", it says.

The document also calls for students to be told about the dangers of 'sexting' - sending explicit text messages - and sharing sexual photos and images.

The new guidance has been drawn up by sexual health charity Brook, the PSHE Association and the Sex Education Forum in a bid to give schools advice on how to teach pupils about the topic.

It comes in the wake of a number of reports warning that sex and relationships education is not up to scratch in many schools and growing fears that children are increasingly exposed to pornography, sexual images and sexual bullying, which could leave them open to exploitation and inappropriate behaviour.

A report published by Ofsted last year found that sex and relationships education (SRE) needs to be improved in more than a third of schools and called for secondary school pupils to learn more about issues such as porn, relationships, sexuality and staying safe, rather than just the ''mechanics'' of reproduction.

Teachers have also spoken out about the issue, with one survey suggesting that s choolchildren are increasingly having sexually explicit conversations with each other after being exposed to pornography. The poll also found that some teachers report an increase in sexual bullying at their school or college.

The new advice, which has been welcomed by ministers, says that schools should teach about the impact of pornography, arguing that there is widespread support from parents for this.

"Teaching should emphasise that pornography is not the best way to learn about sex because it does not reflect real life, and can therefore be worrying, confusing and frightening for young people," it says.

The guidance suggests that young people's experiences of pornography are "distinctly gendered" and it can have negative effects on their attitudes to each other.

Sex education should help young people to understand the influence porn can have on different genders' experience of sex, the advice says.

"Pupils should understand that pornography shows a distorted image of sex and relationships, including 'perfect' bodies and exaggerated sexual prowess. SRE provides opportunities to discuss body image and understand how pornographic pictures and videos are routinely edited and 'photoshopped'", it says.

The guidance also says that some youngsters may be concerned that their use of porn is "compulsive" and that teachers should recommend they talk to a trusted, non-judgemental adult.

The document goes on to say that young people are growing up in a culture where technology and social media are important and there are more opportunities for sharing personal information.

"SRE should encourage pupils to think about what they want others to know and see about them - whether on or offline."

It adds: "Research shows that sexting is of most concern to young people in their early teens, schools should address privacy and boundaries from a very early age in the context of personal safety and abuse. Specific work about 'sexting' should be addressed in SRE as soon as it is identified as a potential issue."

The advice also warns schools that SRE should be inclusive and that teachers should not assume that all relationships are between opposite sexes. It suggests that sexual health information should include lesbian, gay, bisexual and transgender (LGBT) people in case studies, scenarios and role-plays, with pupils exploring topics from different points of view.

Brook chief executive Simon Blake, said: "Young people have been telling us for years that SRE is not relevant to their lives and they want better. We think they deserve better, and in 2011 we asked them what 21st Century SRE should look like. Young people's views from this and other channels over the last two years have influenced this piece of work."

The Government already publishes statutory guidance for schools on SRE and the new document, which was welcomed by Schools Minister Lord Nash, is supplementary advice.

Jane Lees, chair of the Sex Education Forum, said: " The advice we've produced is unflinching and tackles the most challenging issues that can and should be raised in SRE classes.

"For example, how to teach children about sexual consent and address violence in relationships, and how to discuss the misleading portrayal of sexual relationships found in pornography.

"We have received great support for the advice from across health, education and children's services, as well as from parents, teachers and students.

"While we believe that more must be done to guarantee SRE in all schools, we hope this supplementary advice moves us a little closer to helping schools provide what young people need."