Incomplete sex education in schools is leaving children confused about sexual abuse, according to an NSPCC survey.
Children should be taught about where the law stands on sex, says the charity
Some 93% of 2,000 children surveyed said their lessons did not include any information on sexual abuse.
And 82% of them did not know it was illegal for a 30-year-old man to sexually touch a 15-year-old girl.
The NSPCC is calling on the government to make sure 14 to 16-year-olds are taught about sex in the context of law, relationships and peer pressure.
Chris Cloke, the charity's head of children protection awareness, said: "We must arm young people with a clear knowledge of where the law stands on sex and what constitutes sexual abuse."
"That way children will have more confidence to speak out on abuse."
Three-quarters of children who have suffered sexual abuse do not speak out at the time, he added.
The survey was hosted on the National Society for the Prevention of Cruelty to Children's websites, donthideit.com and mykindaplace.com
The survey found that, although 92% of children knew the age of consent, 88% did not consider a 23-year-old woman having a sexual relationship with a 15-year-old as abusive.
And 89% did not regard a 16-year-old having a sexual relationship with a teacher as an abusive situation.
Mr Cloke said: "Sex is a minefield for young people. They face a daily barrage of conflicting messages about sex and yet they don't have the knowledge to guide them through it."
The NSPCC is sending school pupils an information card detailing what sexual abuse is and where they can get advice.
The survey and card are part of the charity's 'don't hide it' campaign, which encourages young people to speak out on all forms of sexual abuse.
The children's charity is also calling for Personal Social and Health Education to be made a foundation subject, and this would include sex and relationship education for 14 to 16-year-olds.