Young people need to be given "the confidence to say no" by being taught about sexual consent at school, a sexual health charity has said.
Spokeswoman for Terrence Higgins Trust, Daisy Ellis, said youngsters need "more than a biology lesson" and deserved more than relying on friends or the internet for advice.
The quality of sex education is "a lottery" with some pupils not taught vital information about boundaries, according to experts.
Sex Education Forum co-ordinator, Lucy Emmerson, said teaching was too "theoretical" and failed to deliver "real-life practicalities".
Education on sexual consent is "woefully lacking" in schools after it emerged that 30% of youngsters are not taught about saying 'yes' or 'no' to sex, according to a new survey.
Research from the Sex Education Forum found three in 10 children had not been taught about consent, which it claimed was "integral" to a good sex education.
Its poll of almost 900 children and young people found that 43% said they had not been taught what constitutes a "good or bad" relationship.
The survey also found that one in three were unsure where they could get help if they were sexually assaulted.
The Prime Minister has admitted he is still "grappling" with how to talk with his own children about the dangers of internet porn, saying:
Prime Minister David Cameron has said sex education in schools must be adapted to address online pornography and called on all parents to learn how to protect their children at home.
Mr Cameron said he agreed with his Education Secretary, Michael Gove, that "wholesale reform" of the curriculum was not required, but said: "We need to make sure we are up to date on the problems of the internet. ... We should be alert to those points and make those changes."
He said the issues concerned parents and school teachers alike, adding: "We are all going to have to get better at understanding all these issues around parental controls."
Laura Perrins, from the campaign group Mothers at Home Matter, has told ITV Daybreak that "any amendment to the children and families bill will disempower parents."
MPs today will vote on the Children and Families Bill to include Sex and Relationship Education in the National Curriculum.
MPs today will vote on the Children and Families Bill. If it passes, Clause 20 will make Review of Sex and Relationship Education a statutory part of the National Curriculum.
Shadow Children's Minister, Lisa Nandy, has told ITV Daybreak: "It's really important [that young children] understand what a healthy relationship looks like."