Teenagers regularly report that lessons are "too biological, too little and too late", while teachers and advisers worry that they cannot cover some topics or answer pupils' questions for fear of running foul of the law, parents or the media.
The losers are the pupils, and particularly those who are concerned about their own sexuality. Gay teenagers - or those who seem different - are more likely to be picked on in school, with official silence on gay and lesbian sexuality compounding their problems.
Yet schools which follow good practice in setting their sex education policy - ensuring both parents and pupils are involved from the outset - may even find that the lessons become one of the most rewarding parts of the curriculum, according to Gill Lenderyou, senior development officer at the Sex Education Forum, an umbrella group of interested organisations.
Many of the worries expressed by teachers and advisers date from 1993 when Aids, HIV and sexually-transmitted diseases were removed from the national curriculum, to appear in newly-compulsory sex education lessons. Simultaneously, parents were given the right to withdraw pupils from such lessons without giving a reason.
A guidance circular from the Department for Education a year later added to teachers' fears by warning that it might not be legally safe to answer individual pupils' contraception queries, and stating that pupils' confidences must be relayed to the parents or headteacher.
These two points are often given as reasons why teachers and advisers are worried about sex education lessons. The notorious Section 28, dating back to 1988 and intended to prevent "promotion of homosexuality", also frightens schools, which as a result often avoid mentioning sexuality in lessons.
According to Ms Lenderyou, the other major stumbling blocks regularly cited by teachers and advisers are parents. "They'll say we can't really do that because the parents won't like it, or that we'd like to do that but we've got different cultures in the schools and we don't want to step on anybody's toes."
She said that none of these reasons should prevent good sex education taking place. Some of the best examples of school sex education she had recently heard of were those taking place in potentially difficult surroundings.
It was important for the school to understand and work with different religions and cultures.
In one North London secondary with a large number of Turkish Muslim pupils, parents were consulted on proposals to start running sex education lessons. They thought it was a good idea, adding that they had never had such lessons themselves.
Ms Lenderyou said: "Right, said the school, we'll give you sex education. They looked at condoms and did a lesson with an interpreter there, so that they could all talk about it properly with the teacher who didn't speak Turkish.
"At that school, the parents are solidly behind the sex education policy. They say: 'We don't want our daughters to go through what we did. It's so frightening. You get married, have a baby, but you know nothing.'" In an East London school where 90 per cent of pupils were Muslim, a very successful policy had grown out of a discussion with the local Iman, who said sex education was in the Koran and he was prepared to teach the oldest boys, with a nurse teaching the girls. Within a few years the lessons were being given to three consecutive year groups, giving the information in a structured way.
It helps to remember that research shows around 96 per cent of parents are consistently in favour of sex education, says Ms Lenderyou.
Advice from the Sex Education Forum:
* Do involve parents, pupils and the local community in formulating the sex education policy; * Do make sure that all points of view are expressed during discussions: explain that there are many different points of view on controversial issues like abortion and that they are all valid; * Do make sure pupils talk about issues at a distance, using role-play rather than their own problems; * Do make sure teachers are properly trained, with good resources * Start by doing what you feel comfortable with; * Do not ignore sexuality: homophobic bullying is common.