Principal Wang Yingjun said about 80 percent of his students are left-behind children, and he warned that the absence of a parental figure creates problems for a child's growth.

Many become addicted to surfing the Internet on smartphones, he said, adding that while boys squander money on cigarettes and alcohol, girls are spending their allowance on junk food or make-up.

"School, parents and the surrounding environment are the three vital ingredients to a child's development," the principal said. "If parents are missing, teachers and school administrators bear more responsibility.

"Teachers now not only need to take care of education but also a students' living conditions and psychological state. That's too much to handle."

Tian Guoxiu, deputy dean of the Capital Normal University's Political Science and Law College and a specialist in child psychology, agreed. She said left-behind children who live without parents can easily feel neglected and suffer self-doubt.

"They will think that they were left behind because they are not important, and gradually they will struggle with confidence and lack a sense of security," she said. "All they have is an empty house, so they can also become silent and unsociable."

Although some people may sympathize with the difficulties migrant workers face in raising children in a city without a hukou, Sun Yunxiao is not one of them. The deputy director of the China Youth and Children Research Center said letting minors live alone is "dangerous and unacceptable" under any circumstances.

"The rules on guardianship for children are just useless words on paper if they are not enforced," he said.

Legal interpretation

The Law on the Prevention of Juvenile Delinquency, introduced in 1999, states parents or other legal guardians should not let minors aged 16 or younger live alone, while the Law on the Protection of Minors, effective since 2007, warns that parents who cannot fulfill their responsibilities should appoint a temporary guardian.

However, there is no legal interpretation about the punishment for those who break the law, experts say.

"Most migrant workers are from less developed areas, so the government needs to look at increasing investment and policies to boost the economy and create more jobs in the provinces that provide that labor force," Tian said.

Plus, authorities need to remove the barriers that prevent children of migrant workers from receiving an equal education in cities, she added.