Campaigners have warned that school sex education lessons are not up to scratch, as a survey found most youngsters get more information outside the classroom.
Around one in seven (13 per cent) teenagers said they learnt most about sex at school, according to a poll by Brook, the sexual healthy charity for young people. Just under one in 10 (9 per cent) said they received most of their information from their mother, father or both parents.
More than a third (36 per cent) said they got most of their information about sex from friends, with 13 per cent saying they learnt about the subject from magazines, books, music lyrics and videos or TV. And 5 per cent said they learnt about sex online from pornography.
Almost half (47 per cent) of the 2,029 youngsters questioned agreed that sex and relationships education (SRE) in their school does not cover what they really want to know about sex. Just over a quarter (26 per cent) agreed that SRE at their school is "non-existent".
More than one in five (22 per cent) of the teenagers questioned rated SRE in their school as "poor" or "very poor", with 40 per cent calling it "average" and 34 per cent "good" or "very good".
Nearly four in five (78 per cent) said they had not been given the opportunity to influence what is covered in their SRE classes, with 72 per cent saying pupils should be asked what they want to be taught. More than half (52 per cent) said relationships and emotions are not discussed enough in their SRE classes.
Jules Hillier, Brook deputy chief executive, said: "Young people in Britain deserve honest, useful information about sex and relationships but SRE in UK schools is failing them.
"Learning about sex and relationships is a crucial life skill and by letting teenagers leave school ill-informed we are letting them down. We are calling on young people to seize the opportunity to make their voices heard by telling us what they think 21st century SRE should cover, to better meet their needs."
A Department for Education spokesman said: "We are carrying out a wide, internal review of the personal, social, health and economic (PSHE) curriculum to strengthen classes to address weaknesses reported by Ofsted last year.
"We are simplifying the statutory guidance on sex education to focus on relationships, positive parenting, and teaching young people about sexual consent. We have launched a public call for evidence - and will consult on firm proposals in due course."
Only last month, a survey by the family education trust raised questions over the teaching of sex education and students told the Huffington Post of their experiences and concerns about the issue.