BEIJING -- Most children remember their school days fondly, but for students of a school in Youxian County in central China's Hunan province, it is a time they would prefer to forget.

A math teacher from the county is awaiting trial over charges of sexual assault and sexual battery after 23 girls came forward to say that they had been abused by the man over a period of three years.

School and education officials were aware of the abuse but no one informed the police, Xinhua reporters discovered.

Moreover, the abuser appears to have targeted particularly vulnerable victims, as 18 of the 23 girls were "left-behind" children, those whose parents are migrant workers in other cities. It is on this basis that investigators put forward the argument that he chose them because they "would keep their mouths shut."

"We were ashamed to tell our grandparents. Every time my mother called, she would tell me to study hard and listen to my teachers. I could not bring myself to tell her what was happening over the phone," one of the victims told Xinhua.

More than 60 million children are considered left-behind in China, and they are at greater risk of falling victim to sexual assault or physical abuse, lawyers and children rights campaigners agree.

Unfortunately, saving face and reputation often trump a desire for justice, especially for older care-givers, said Li Huiye, a lawyer at Tiandiren Law Firm, Hunan.

"Silence and private settlement is the preferred solution, instead of conviction," said Li, adding that such a culture enables sexual predators to commit crimes against children.

From 2013 to 2015, at least 968 incidents of sexual assault against minors, involving 1,790 victims, were reported in Chinese media, according to the sexual violence prevention center Girls' Protection Foundation.

As Children's Day falls on Wednesday, campaigners want to bring the subject of abuse out of the shadows, and protect children from violence, exploitation and abuse.

Boundaries, education

Sex education can help prevent sexual abuse. Children are not being taught about their bodies, their individual right to privacy or how to define boundaries. This lack of understanding only exacerbates the already entrenched stigma attached to victims of abuse, making children even more vulnerable, child protection experts have said.

Fei Yunxia volunteers for Girls' Protection, it is her responsibility to visit schools and teach children about their bodies and rights. During each presentation the mood is often the same: The students lower their heads, avoid eye contact, and, more often than not, an awkward silence hangs heavy in the hall.

During a recent presentation in Gaoqiao Village in Hunan, she spoke to the students about their bodies and personal boundaries, as children that understand their bodies are more likely to know if certain interactions are appropriate or not. Meaning that if they are being abused, they understand that it is wrong, or if someone tries to push their boundaries in the future they can prevent it.

Girls' Protection, which was founded in 2013, funds lectures, campaigns and research to prevent sexual violence against children.

"Before me, nobody told these students about their bodies, sex or how to protect themselves from harm," said Fei.

In the last year, Fei has given lectures to 2,000 students in four schools in Hunan, and she wants to reach out to more. There are about 6.9 million students in elementary schools and junior high schools in Hunan.

"It is just hard to get support from schools or education authorities. They think the topic is 'sensitive' or 'unspeakable,' and others do not think sexual abuse deserves special attention," she said.

There is no sex-ed program on the prevention of sexual abuse, and many schools would not even consider approaching the subject, said Sun Xuemei, one of the founders of Girls' Protection.

Last year, Girls' Protection polled more than 4,700 students, and 40 percent of respondents could not identify their private parts. It also interviewed 363 parents, and 40 percent never spoke to their children about sexual abuse at all.

Many parents avoid sex education completely.

"They either think it is too early to talk about sex, or talking about it would corrupt their children. It seems that sex is just not a topic suitable for public conversation," said Sun.

Someone to talk to

Xiao Yun is a survivor-turned-campaigner with Girls' Protection. She was sexually abused by her neighbor when she was eight years old.

"There is hardly anyone for victim to turn to. The security net around children is far too porous," she said.

"After a lecture at a school, I received a text message from a girl who said she had a sexual experience against her will, but when I called her back, her phone was dead," she said.

According to the People's Public Security University of China, for every report of sexual abuse there may be at least seven unreported incidents.

"There may be thousands more victims too scared to tell," she said, "they need to know that there is counseling offered by schools and hospitals."

Ye Qian, a psychologist in Beijing said many underage victims still bear the scars of abuse in adulthood.

"Sexual abuse may affect mental wellbeing, causing irreversible harm," she said.

"The victims should speak out, ask for help, and seek justice," Ye said, adding that it was "the responsibility of parents, schools, hospitals, police and communities to offer safe environments for this."

Legislation required

Many believe that the compensation awarded to the underage victims of sexual abuse does not reflect the severity of the crime.

A kindergarten student in northeast China's Jilin Province was awarded just 1,000 yuan in damages after the court threw out the psychological damage claims filed by the victim's family. The child's abuser was the husband of the owner of the kindergarten.

In China, only females can be victims of rape, and those found guilty of assaults of this kind against boys usually receive light sentences.

"If a boy is raped, the offender faces a maximum five-year sentence as they are convicted under the crime of obscenity. If the victim is a girl, and the damage is severe, the abuser could get death," said Li Ying, a Beijing-based lawyer.

Of the 340 reported sexual assault cases in 2015, at least 20 involved boys, around 60 individuals.

"Boys and girls must have better legal protection," Li said.