Almost a third of young people said they did not learn about sexual consent in sex education lessons, says a survey from the National Children's Bureau.
Sex education in England's schools often ignores such real-life issues, says the children's charity.
It also found one in three did not know, or was unsure, where to get help if they were sexually assaulted.
The Department for Education says it has given extra funds for training, and to promote the teaching of consent.
The survey was conducted by the Sex Education Forum, which spoke to almost 900 children and young people aged between 14 and 25.
A third of them said they had not been taught about the notion of consent, and only 43% said they had learned about what was "good and bad about a relationship" in lessons.
Sex and relationship education is compulsory in maintained secondary schools.
But the Sex Education Forum's co-ordinator, Lucy Emmerson, says the findings suggest lessons in schools are failing to build necessary life skills.
"Teaching is often too theoretical," she says. "It fails to deal with the real-life practicalities of getting help, or building the skills for pleasurable, equal and safe relationships."
She is calling for every school to have a planned programme which teaches pupils about "bodily boundaries, caring for one another, feelings and emotions".
A Department for Education spokeswoman said it had given £100,000 to the Personal Social Health Education (PSHE) Association to help schools improve staff training and promote the teaching of consent.
"Schools must teach pupils about how the law applies to sexual relationships so that they understand concepts such as sexual consent," she said, adding that the department recently consulted on a guide for teachers on the concept of consent.
The Association of Teachers and Lecturers believes the survey reveals a "dire need" for all young people to have good sex and relationships education to keep them safe and allow them to make informed decisions.
The association's general secretary, Mary Bousted, said sex and relationship education should be covered in a "meaningful way, and not just appear sporadically on the timetable".