Twenty teenage school girls have been caught offering sexual services for money through online network in China's commercial hub Shanghai, local prosecutors said.

Three of the girls have been charged with organising prostitution and procuring, after they persuaded classmates to join them and took a commission for helping them contact customers.

According to Han Konglin, the prosecutor from the People's Procuratorate of Zhabei District in Shanghai, the prostitution ring was established in 2009 when the three girls started offering sex service and looked for more girls to join them.

The girls ran the business online, asking their customers whether they needed a young girl "to play with" and arranging an appointment, usually at hotels late at night, state-run China Daily reported today.

The girls were caught after a customer reported the theft of a valuable watch to the local police after an "appointment" with one of the girls.

Han said that many of the girls joined the ring only to earn some spending money and did not realise they were breaking the law.

"I don't want to be a worker like my parents, living a hard life to earn money," one of the girls told prosecutors.

"I didn't mean to get my friends into trouble. They all love to do the job because our parents never give us enough pocket money to spend, but money is needed in every way," another girl said.

Huang Hongji, director of the Shanghai Youth Research Centre, said an increasingly materialized society is changing teenagers' sense of values by giving them the belief that money is essential if they want a better quality of life.

"All the teenagers involved in the case are students from vocational or high schools where they didn't receive enough moral education, they didn't know that prostitution is not the right way to make money," Huang said.

There has been a rapid increase in the number of teenage prostitution cases in cities all over the country in the last three to five years, the daily said.

"As more parents and schools fail to offer enough sex education to teenagers, the number of teenagers involved in prostitution cases has grown," said Qian Yuanchun, a lawyer who specializes in teenage-related cases at Shanghai Yuanwen Law Firm.