In neat rows, the Pakistani girls in white headscarves listened carefully as the teacher described the changes in their bodies. When the teacher asked what they should do if a stranger touched them, the class erupted.
"Scream!" one called out.
"Bite!" another suggested.
"Scratch really hard with your nails!" a third said.
Sex education is common in Western schools but these ground-breaking lessons are taking place in deeply conservative rural Pakistan, a Muslim nation of 180 million people.
Publicly talking about sex in Pakistan is taboo and can even be a death sentence. Parents have slit their daughters' throats or doused them in acid for crimes as innocuous as dancing at a wedding or looking out the window.
Almost nowhere in Pakistan offers any kind of organised sex education. In some places it has been banned.
But teachers operating in the village of Johi in poverty-stricken Sindh province say most families there support their sex education project.
Around 700 girls are enrolled in eight local schools run by the Village Shadabad Organisation. Their sex education lessons - starting at age 8 - cover changes in their bodies, what their rights are and how to protect themselves.
"We cannot close our eyes," said Akbar Lashari, head of the organisation. "It's a topic people don't want to talk about but it's fact of our life."
Facts of life
Lashari said most of the girls in the villages used to hit puberty without realising they will begin to menstruate or they got married without understanding the mechanics of sex.
The lessons even teach the girls about marital rape - a revolutionary idea in Pakistan, where forcing a spouse to have sex is not a crime.
"We tell them their husband can't have sex with them if they are not willing," Lashari said.
The lessons are an addition to regular classes and parents are told before they enroll their daughters. None has objected and the school has faced no opposition, Lashari said.
The eight schools received sponsorship from BHP Billiton, an Australian company that operates a nearby gas plant, but Lashari says sex education was the villagers' own idea.
Teacher Sarah Baloch, whose yellow shalwar khameez brightens up the dusty school yard, said she hoped to help girls understand what growing up meant.
"When girls start menstruating they think it is shameful and don't tell their parents and think they have fallen sick," she said.
Baloch teaches at a tiny school of three brick classrooms. A fourth class is held outside because there are so many girls.
Three girls cram into each seat made for two, listening attentively to Baloch. One flashcard shows a girl stopping an old man from touching her leg. Other cards encourage girls to tell their parents or friends if someone is stalking them.
The girls are shy but the lessons have sunk in.
"My body is only mine and only I have the rights on it. If someone touches my private parts I'll bite or slap him in the face," said 10-year-old Uzma Panhwar defiantly as she blushed.
The lessons also cover marriage.
"Our teacher has told us everything that we'll have to do when we get married. Now we've learned what we should do and what not," said Sajida Baloch, 16, staring at the ground.
Ahead of its time?
Some of Pakistan's most prominent schools, including the prestigious Beaconhouse School System, have been considering the type of sex education practised in Johi.
"Girls feel shy to talk to their parents about sex," said Roohi Haq, director of studies at Beaconhouse.
There is definitely demand. Lahore-based Arshad Javed has written three books on sex education and said he sells about 7,000 per year. None are sold to schools.
But not everyone agrees with the lessons, partly because young people were not supposed to have sex before adulthood. Recently the government forced the elite Lahore Grammar School to remove all sex education from its curriculum.
"It is against our constitution and religion," said Mirza Kashif Ali, president of the All Pakistan Private Schools Federation, which says it represents more than 152,000 institutions across the country.
"What's the point of knowing about a thing you're not supposed to do? It should not be allowed at school level."
In neighbouring India, many government schools formally offer sex education but Pakistani government schools have no such plans. Nisar Ahmed Khuhro, the education minister for Sindh province, was shocked to hear of the lessons.
"Sex education for girls? How can they do that? That is not part of our curriculum, whether public or private," he said.
But Tahir Ashrafi, who heads an alliance of moderate clerics called the Pakistan Ulema Council, said such lessons were permissible under Islamic law as long as they were segregated and confined to theory.
"If the teachers are female, they can give such information to girls in the limits of Sharia," he said.