Hong Kong (CNN) -- Like millions of China's migrant workers, Chen and his wife left their only daughter behind when they went to seek work in the southern boomtown of Guangzhou three years ago.
But Chen, who once thought his home was a sleepy and safe place for his nine-year-old to grow up with her grandmother, had that belief shattered when he received pleas for help in November last year.
"She kept calling us, begging us to come home. She said she wasn't feeling well, that she was always feeling down, and that it hurt," Chen, who only gave his first name, told CNN by telephone from his hometown of Xiangxiang in central China's Hunan province.
"She said it hurt 'down there.' We knew instantly that things were not right."
With much coaxing and tears, his daughter finally told him what was wrong: She had been raped by her teacher.
Afterwards he gave her two new notebooks, Chen said.
She was one of the teacher's five alleged victims -- all of whom were under 14 and went to the same school, state media reported.
"Never did we imagine that harm would come at school. There is no safety to speak of if school isn't safe," said Chen.
The case has shone light on the plight of China's estimated 61 million "left-behind" children -- one in five nationally -- that grow up without one or both of their parents, according to statistics from the All China Women's Foundation.
Around 30 million children under 18 have no parent at home and two million fend for themselves with no adult guardian, the figures show.
It's a heart-wrenching consequence of what has been described as the greatest human migration of all time -- some 250 million have left China's poorer inland provinces to forge a living in coastal factory towns and cities.
These families only reunite a few days each year—for the most part during Spring Festival or Lunar New Year, which began on January 31 this year.
Ye Jingzhong, the author of "A Different Childhood: Children Left Behind in Rural China," says children like Chen's daughter are easy targets for sexual predators.
He says they lack the attentive care and supervision of parents, and are often entrusted to relatives and less devoted guardians.
"Materialistic values are seeping into the countryside and -- without parents to tell them right from wrong -- they (children) easily fall prey to bait like candy and new cell phones," Ye told CNN.
According to state news agency Xinhua, sexual assaults on left-behind girls in rural areas account for the majority of reported assaults. In some places, such as Huazhou in Guangdong province, 94% of cases involve left-behind children, the report added.
Xinhua said authorities in the town where Chen's daughter attended class had ordered schools to hold "safety meetings" to improve educational supervision and strengthen teachers' ethics. Schools should also investigate what factors leave students vulnerable to sexual abuse and how to guarantee the safety of all students, especially girls, it added.
China's left-behind generation is not just vulnerable to serious crimes like sexual assault and harassment -- long-term separation has emotional costs for both children and parents.
"I wish they could come home during the Spring Festival. I somehow feel like an unwanted burden in our family," 12-year-old Xiaoli told the authors of a new report on China's left behind children by the Centre for Child Rights and Corporate Social Responsibility (CCRCSR).
The report found that 82% of the 877 migrant parents it surveyed regarded themselves as inadequate.
A-ying, the 33-year-old mother of two left-behind children, said her son is unwilling to take her calls.
"The reason he dislikes us is because he blames us for not taking care of him, because he did not get proper care. Living together would have helped make our relationship more affectionate," A-ying was quoted by the report as saying.
Sanna Johnson, the Beijing-based executive director of the organization, says the effects are far-reaching.
"When you have 61 million children who cannot relate to their parents, it is very traumatizing for a society."
Ye shares her concerns, imagining a society that has grown up emotionally detached, unable to reach out in times of need.
"It's hard to say what the direct consequences will be when the left-behind generation comes of age, but I'm afraid it will not work toward a 'harmonious society'," he said.
CNN contacted China's Ministry of Education and is awaiting a response to questions on what action authorities are taking to protect "left behind" children.
China has been recruiting more social workers to provide counseling in rural areas, and government spending on social services rose an average of 24% from 2008 to 2012, according to 2013 figures from the Organization for Economic Co-operation and Development (OCED).
Rural schools are providing more boarding facilities and the government has signaled it will relax restrictions that make it difficult for migrant workers to bring their children to smaller cities, although larger cities like Beijing are not included in the proposed changes.
However, Ye says things will only start looking up for China's left-behind children when officials take their plight seriously.
"It's time to start thinking about what rural China really needs with heartfelt concern."
Back in his home village, Chen is trying to rebuild his relationship with his daughter. He says she has retreated into herself, flees from strangers and often sheds silent tears.
After the Lunar New Year, he plans to take her with him to Guangzhou, where he works as a decorator, and her mother will quit her job to look after her.
Chen refused to accept the 10,000 yuan ($1650) compensation fee he said he was offered by the school. He is furious that local authorities have not made an apology and says he wants to see his daughter's rapist "rightfully sentenced."
According to Xinhua, a man has been detained in connection with the case since December but no date has been set for a trial.
Chen fights a lonely war, and imagines the families of the other accusers are happy to take the money and do their best to forget.
He told CNN that he intends to "create a scene" at the school after the Chinese New Year vacation if the semester starts and the alleged perpetrator is not behind bars.
"There's no saying -- maybe we will have to use violence to make sure other children do not go through what our daughter did."
CNN's Katie Hunt in Hong Kong and CNN's Beijing bureau contributed to this report