Pupils are left uninformed about sex because the "sensitive" topic is often glossed over in the curriculum, according to a report by Ofsted, the government's schools inspectorate.
Teachers' prudishness even resulted in complaints from children and parents, who felt there needed to be more teaching of sex at an earlier age.
At one in three schools pupils' knowledge of sex and relationships was rated "no better than satisfactory", while in three schools it was "inadequate".
In primary schools pupils were often left with only a "partial understanding" of sex and a number of pupils, mainly girls, told inspectors they needed more knowledge and information earlier than the age of 11.
Many secondary school pupils told inspectors their sex education was "too late and too limited to be of much use", while parents at one primary school successfully campaigned to have the sex education programme brought forward from year six to year five, meaning it was taught to children as young as 10.
In other schools children were given a good understanding about the mechanics of sex and puberty, but not taught about emotional development, relationships or marriage.
The report said sex had been "squeezed out" from the curriculum because "since the topics were particularly sensitive, some teachers felt uncomfortable in teaching such demanding themes."
It said many children pick up information on sex from TV dramas, adding: "This highlights the need for sex and relationships education in schools."
Plans by Labour to add sex education to the National Curriculum and remove parents’ right to withdraw children from lessons when they turn 15 were scrapped ahead of the general election.
The report identified "serious gaps" in the personal, social, health and economic education curriculum at 12 secondary schools, mainly related to sex, drugs, mental health, sexism and homophobia.
Teaching of the dangers of drugs and alcohol was criticised, with pupils in half of secondary schools only retaining a "rudimentary" understanding of the physical effects of drinking.
In a third of schools students' knowledge of drugs, alcohol and tobacco was one of the weakest aspects of their personal, social, health and economic education, with some teachers failing to make clear that drinking alcohol is illegal below the age of 18.
Many students believed heroin and cocaine were the drugs responsible for the most deaths each year, not realising that the number of deaths caused by alcohol and tobacco are significantly higher.
A spokesperson for the Department for Education said: “We want all young people to benefit from high quality PSHE teaching and we will take this report’s findings into consideration as we continue to look at the curriculum across the board.
"We will make an announcement on future plans for the curriculum as a whole in due course.”
--By Nick Collins,23 Jul 2010