A recent survey of 2,700 secondary school students found that the number of them engaging in "sexting" has doubled from a year ago.

It found that 4.2 per cent of upper secondary students and 1.9 per cent of lower secondary students had sent lewd messages, photos and videos, or posted risque content via their mobile phones. This is up from 2.3 per cent and 0.8 per cent respectively last year.

Said Mr Chong Ee Jay, manager of Touch Cyber Wellness, a voluntary welfare group that teaches Internet safety: "The sexting trend started in the United States and Britain in 2010. Two years ago, we started hearing about one-off cases here."

According to the survey conducted from January to May, more teens are aware of sexting and what it means.

About 71 per cent of upper secondary students and 53 per cent of lower secondary students said they understood the term, up from 62 per cent and 41 per cent respectively last year.

Mr Chong said he is concerned about the jump in the number of students involved in sexting, although the figures are still small.

"Many of these cases happen in the context of boy-girl relationships, not with strangers," he said.

"But we tell students that there's no guarantee of privacy for anything that is sent via tech devices, and there are consequences if the photos are leaked."

He recalled a case last year where a Secondary 4 girl was asked by her boyfriend to send him nude and provocative photos of herself.

"She wasn't comfortable with it but he reassured her that they were only for his own viewing," he said.

But the boy, who is also in Secondary 4 and at the same school, showed the photos to his friends, and the girl was called a "slut".

Mr Chong added: "At that age, teens are experimenting, and they are young and impressionable, especially in relationships.

"It could happen in the heat of the moment, but we remind students that their personal information can be used against them once it's out of their control."