In an age when "extremely violent and sadistic imagery is two clicks away" school sex education is struggling to keep pace, a study suggests.
Pornography can distort children's attitudes to sex said Deputy Children's Commissioner Sue Berelowitz.
Urgent action is needed to develop children's resilience to extremely graphic types of porn argues the study.
The government said its curriculum changes would teach children from the age of five to stay safe online.
The report, led by the University of Middlesex and commissioned by the Office of The Children's Commissioner, suggests some children are exposed to pornography while still at primary school, and the proportion increases with age with "a significant proportion of children and young people" viewing pornography.
Lessons on relationships should start in primary school, it argues, while relationships and sex education should be compulsory in all schools and include time for pupils to discuss the impact of pornography.
Some types of online porn are "very different" to what today's parents may have seen as children, said Ms Berelowitz.
"Just a few clicks away on any mobile phone, on any tablet for example, children can find really graphic depictions of extreme and violent sexual acts."
The report suggests that pornography can affect attitudes and behaviour among children and young people.
It can lead to more sexually permissive attitudes, more casual sex, sex at a younger age, and the belief that women are sex objects with males dominant and females submissive, suggests the study.
There is a correlation between children and young people who use pornography and "risky behaviours" such as anal sex, sex with multiple partners and using alcohol and other drugs during sex, say the authors.
The authors draw a distinction between "being exposed to pornography" - being forced to watch it or stumbling upon it online - and deliberately "accessing" it.
Young men and boys are more likely than young women and girls to do both, say the authors.
"Boys and young men generally view pornography more positively and state that they view it primarily out of curiosity while girls and young women generally report that it is unwelcome and socially distasteful."
The findings are based on interviews with young people in England, plus analysis of 276 previous academic papers on young people and pornography.
The report urges the Department for Education to ensure that all schools, including private schools, faith schools, colleges and academies, "deliver effective relationship and sex education".
It also notes emerging evidence that young people are dissatisfied with the sex education they are receiving and "increasingly" draw on pornography for education and information on sexual practices.
The authors say the sex education curriculum needs to be more relevant to young people's lives and include pornography.
They also call for more emphasis on relationship education in secondary schools.
"We think it's really important that the curriculum includes pornography to help build children's resilience to what they are seeing on the internet - to help them differentiate between what they are seeing and good healthy relationships which are not about submission and not about being forced," said Ms Berelowitz. .
She added that parents needed to recognise the effect of pornography on their children.
Lucy Emmerson, of the Sex Education Forum, welcomed the call to boost sex education: "Good quality sex and relationships education takes place when the subject is given enough time in the timetable, is included in every year of schooling and is taught by trained teachers.
"It is then possible to introduce children to key themes such as consent, gender, body image, relationships and sexual behaviour. These themes can be introduced in an age-appropriate way and then built on year by year."
A DfE spokeswoman said sex and relationship education was already compulsory in maintained secondary schools but it was up to primary schools to decide whether to teach it.
"We are strengthening the curriculum so that, from the age of five, children will be taught how to stay safe online. Schools can already teach children about the dangers of pornography provided all lessons are age-appropriate and follow the correct guidance.
"The UK Council for Child Internet Safety is already working with internet service providers to make it easier for parents to protect their children from harmful material online."
Siobhan Freegard of the parenting site Netmums called the report a "wake-up call".
"The consequences of young kids viewing horrific porn are only just becoming apparent with sex attacks by underage kids doubling in some areas in just 12 months and kids as young as five being assaulted.
"Doing nothing isn't an option and we need to implement everything in the report and more to keep our kids safe."
A joint statement by End Violence Against Women Coalition and Rape Crisis added: "This report provides further strong evidence of the need for schools to be required to teach young people about sexual consent and how to deal with pornographic and violent imagery they see online, in music videos, adverts or elsewhere."