Pupils should continually receive advice about sex and relationships while they are at school, leaders of charities and a teaching union have said.
The call comes after a survey by the UK Youth Parliament showed that nearly half of British youngsters had not been taught about teenage pregnancy and would not know where to find their local sexual health clinic.
Now a group of experts, including Nick Partridge, chief executive of the Terrence Higgins Trust, Steve Sinnott, general secretary of the National Union of Teachers, and Dame Mary Marsh, chief executive of the NSPCC, have expressed their concerns in a letter to The Times.
They said: "These figures ... may go some way to explaining disproportionately high rates of teenage pregnancy and sexually transmitted infections in this country.
"We believe that all children and young people are entitled to receive Sex and Relationships Education (SRE) as part of the statutory provision of Personal, Social, Health and Economic Education (PSHE Education) in schools.
"No school should be able to opt out of delivering good Sex and Relationships Education to their pupils (including primary schools, faith schools and academies), which should be taught throughout a pupil's time in education."
Responding to the letter, schools minister Jim Knight said: "The UK Youth Parliament's findings are disappointing.
"Nevertheless, our Teenage Pregnancy Strategy is working - teenage pregnancy rates are at their lowest for 20 years.
"We made a commitment to reduce the teenage conception rates and to improve the quality of life for all young people, we are delivering on that."