Kathy French, a specialist at the Royal College of Nursing, this week criticised the quality of information given to pupils, saying it differed vastly between schools.
She said lack of openness with children at an early age was to blame for exposure to diseases later in life.
According to the latest figures, one in 10 girls aged 16 to 19 has chlamydia. The Department of Health says, for all age groups, chlamydia increased by 9 per cent between 2002 and 2003, syphilis by 28 per cent and genital warts by 2 per cent.
Addressing a Royal College of Nursing conference of private school nurses in London, Mrs French said: "Sexual advice should start at a very young age. There should be an openness with young children about where they come from as early as possible, and the level of advice should be built upon throughout primary and secondary school.
"The problem is not everyone agrees with that - it is often seen as polluting young people's minds. Many teachers are even blocked by parents and governors who see sex education as wrong."
Mrs French, who sits on a government independent advisory group on sexual health and HIV, said: "Advice should be delivered by nurses and people from sexual clinics, rather than teachers."
A report by the United Nations Children's Fund in 2002 said the UK had the second-highest teenage pregnancy rate in the developed world. It criticised the poor quality of sex education.
The Church of England admitted some "squeamishness" still existed in certain church schools towards sex education. But a spokesman said the church actively supported teaching about sex and sexual relationships "as long as it is in the context of marriage".