That is why the teacher talking about adolescence in the class was not an employee of the school, but a visitor from an educational organisation, Jidnyasa, which seeks to fill gaps in the curriculum by teaching pupils about such matters as drug addiction, stress and, perhaps most importantly, sex.

Sex education has never been a strong point in Indian schools, but earlier this year Maharashtra state - Mumbai is the capital - banned it altogether. The main cause was the new Adolescence Education Programme (AEP), recently devised jointly by the government body responsible for curbing the spread of Aids, the National Education Ministry and children's fund Unicef.

Several legislators felt the language and illustrations used in the AEP were too explicit for children. The fact that Maharashtra had one of the highest rates of HIV infection in India was felt to be a lesser concern. Sex education was banned, and five other states soon followed Maharashtra's example. Many religious conservatives, who had argued the AEP was a western import that would corrupt young minds, cheered the ban.

'The programme is being pushed through schools and colleges in an attempt to create a homogenised culture,' said Nusratullah Afandi, assistant secretary of the Jamaat-e-Islami, an Islamic cultural organisation. 'India has its own culture. And anyway, sex is instinctive. It is not necessary to teach children about it.'

Many parents and teachers disagree. They believe that a good grounding in sex education is crucial for children who grow up watching sex-heavy western television channels - but living in a predominantly conservative society where sex is rarely discussed.

'Talking about sex in India is taboo, and when anything becomes taboo myths about it spread easily,' said Sandeep Shetty, who gives seminars in Mumbai on sex for young people - and adults - covering subjects such as sexually transmitted diseases and date rape. 'Like most men and women of my age - I'm 42 - I got a lot of misinformation about sex. My generation is still so shy about discussing it.'

A recent poll by the magazine India Today revealed that one in four Indian women between the ages of 18 and 30 in 11 cities had sex before marriage. One in three said she was open to having a sexual relationship even if not in love.

Rajan Bhonsle, who runs the Mumbai-based Heart to Heart Counselling Centre, which gives sex education training to teachers (the ban on sex education does not apply to private schools), said there is an urgent need for the dangers of casual sex to be taught in schools.

Although the number of people with HIV/Aids in India is, at 2.47 million, less than half of previous estimates according to a new UN-backed government survey released this month, India has the third highest caseload after South Africa and Nigeria. The federal government backs sex education to curb the spread of HIV/Aids, even if state governments reject it.

'We are hypocrites,' Women and Child Development Minister Renuka Chowdhury told reporters this month at the inaugural meeting of the National Women's Forum of the Indian Network for People Living with HIV/Aids. 'We have a population of 1 billion and don't want to talk about sex.'

Dr Bhonsle said that as well as HIV, children must also be taught about abuse. 'Sex abuse in India is so common that unless children learn to identify right from wrong, how can it ever be stopped?'

Despite the efforts of the central government and other champions of sex education, there are several barriers to be overcome before this subject makes it on to the curriculum of all Indian schools. As the ban in Maharashtra has shown, conservative right-wing political groups in India are quick to make capital out of controversial issues such as sex education.

While many in India, especially the young, embrace modernisation and western values, an increasingly vocal moral police attempts to reach out to the more traditional sections of society by noisily protesting everything from televised kisses to paintings of naked Hindu gods.

The radical fringe of the Bharatiya Janata Party (BJP), which played a leading role in the banning of sex education in Maharashtra, exploits this climate, as the rest of the party tries to reach out to the moderate centre of Indian politics.

But the issue of sex education in India is not simply about the clash of values. There is a genuine misunderstanding among many Indians about what sex education actually involves. 'You talk about sex education and people immediately think you are talking about the act and only the act,' said Mr Shetty. 'It may be that in India we need to find a new term to cover this broad area.'

Jidnyasa, for example, is careful to use terms like 'adolescent sensitisation programme' and 'family life education'. Introducing sex education across all schools in India is made harder by differences in sensibilities and knowledge between the rich and poor.

'The more affluent have access to the internet to look up anything they may be wondering about, and their parents tend to be more broadminded,' said Mr Shetty. 'A little kid in a slum, whose parents would never talk about sex, might have a very limited understanding.'

Dr Bhonsle said sex education should vary from school to school, rather than expecting one programme - like that introduced by the government - to work everywhere. Though private schools in Maharashtra are exempt from the ban on sex education, many have watered down the new programme.

Others, like Sahyadri College, one of thousands of inexpensive private schools in India that cater to the working class, do not offer sex education at all. 'We've never learned anything like that before,' said 14-year-old Vishakha as the bell rings at the end of the adolescence class. 'It's normally all mathematics, languages and history.'

Even in Mumbai's more expensive private schools, sex education is felt by many to be patchy, at best. Balu Sudha, who teaches at Walsingham House, a private girls' school in Mumbai, took a training course at the Heart to Heart Centre because she felt ill-equipped to deal with pupils' wide-ranging questions about sex. She also felt the options offered by the school in this category of education were inadequate.

'There is a visitor who comes in to lecture the pupils about sex education but only two or three times a year,' she said. 'I would prefer it to be at least every month, with more issues discussed and more input from pupils. Children today know about sex - the media doesn't leave much to the imagination. But they need to be able to talk about the issues around sex - peer group pressure, emotional issues and so on. Otherwise they'll have the physical knowledge but not much else.'

In the absence of such education in schools, a growing number of middle-class parents have taken it upon themselves to educate their children about sex. Neelu Grover, whose 16-year-old daughter attends a good private school in the city, said she became aware of a lack of sex education in her own upbringing when her daughter began to question her about sex.

'I consider myself a modern, enlightened sort of mother, but I found that when my daughter started asking me questions I knew very little,' she said. 'She wasn't finding out what she needed to know at school, either. In India, teachers are not ready to accept that, hello, teenagers might be curious about sex. The teaching is completely outdated and inadequate.'

While many schools in India fail to teach sex education successfully, and even more ignore the subject altogether, there are examples in the nation of how sex education can be taught in schools without offending or upsetting anyone.

Ironically, one of the best providers of sex education in Mumbai is the Catholic Church. The Archdiocesan Board of Education, which runs 113 schools in the city, has taught a comprehensive sex education programme hailed as a great success by parents and teachers - many of whom have staunchly conservative moral views.