In May 2007, Liu's team launched sex education classes at Xingzhi Primary School, a private establishment for the children of migrant workers in Beijing.

"Home visits we conducted revealed a number of problems or threats to the health and safety of that group of children. Some students were extremely aggressive as a result of a lack of parent-child time, while others were living in poor conditions with sanitation problems and safety risks," said schoolteacher Shen Guixiang.

A small number of the students live with their grandparents, who don't know how to discuss sexuality and are unable to provide guidance.

Shen recalled visiting a 12-year-old girl whose mother had died. The girl was living with her father in a dormitory for construction workers, and sleeping in the same room as several adult males.

"After I left, I couldn't sleep. She might have been sexually assaulted at any time. I knew we should do something before anything awful happened," she said.

Liu has conducted research among parents and students. About 15 percent of parents said they had discussed sex with their children, but they only focused on puppy love and how to deal with it.

When Liu interviewed a number of junior high school students, she was astonished by their ignorance.

"Some girls said they could get pregnant after sitting on a seat that had recently been occupied by a boy, while others said kissing or sleeping in the same bed could also result in pregnancy," she said.

The lack of sexual knowledge at an early age can lead to problems such as bullying, which can occur when children hit puberty, she said.

According to Liu, the mental functions of pubescent children develop rapidly, which can result in instability.

Therefore, children need support from adults they trust, and talking about sex at an early age can be beneficial because it can establish close relations between children and their parents.

Moreover, LGBT children are often targeted by bullies. "That's why I suggest that children should be introduced to sex education classes as early as possible, because they will understand the concepts of equity and mutual respect at an early age," she said.

Crucial role

Teachers play a crucial role in sex education. The entire process - from training teachers to pre- and post-class interviews with students, to teaching plans - is under the guidance of Liu's team.

"Prejudice and shame about topics related to sex have been eliminated. We cannot give children a stereotypical message," said Wu Ling, who has taught sex education at Xingzhi Primary School for six years.

She said Liu's methods include loudly reciting the scientific names of the reproductive organs, which are usually described in euphemistic terms.

Lively learning methods are encouraged, and classroom activities include discussing the illustrations, telling stories, asking questions and sharing ideas.

The teachers invite the students to invent role-playing scenarios to hone their abilities to deal with a range of situations and problems.

Students are also encouraged to conduct research at home, such as finding out how their parents met and how they formed their family. These 'family stories' are then shared in class.

Before the start of each class, Wu carries out a survey of the content she will teach.

A recent course focused on menstruation and nocturnal emissions.

Among the 46 students in her fifth grade class, the parents of just five of the girls had explained the basics of sex, and only one boy said his father had spoken to him about nocturnal emissions after finding a stained bed sheet. "The father simply asked his son not to tell anyone else," Wu said.

Ge Yaqing, who graduated from Xingzhi Primary School in 2015, is one of the few school students in China to have received systematic sex education for six years.

The 14-year-old is now a senior student at the Gu'an No 2 Primary School in Hebei province.

She said fewer than 10 of the more-than 1,300 students in her grade received sex education in primary school.

"Many of my classmates didn't know how to ease menstrual pains, and some had even called paramedics. They believe sex should not be discussed at any level," she said.

"When I taught them the right way to handle menstruation and nocturnal emissions, they were astonished and asked why a primary schoolteacher had taught us such things. They wanted to know why we weren't ashamed to talk about it."