In Houston, Texas, thousands of students are returning to school to discover a new rule: no "sexting" -- the distribution of nude or semi-nude photographs or videos by text message.

In Houston, one of the biggest school districts in the United States, education authorities decided to adopt the new rule before some 200,000 students returned to classes after their summer vacation.

"Some principals raised the issue, so we included a provision saying sending, receiving, possessing sexually suggestive messages is forbidden," said Hans Graff, assistant general counsel at the Houston Independent School district.

"They were reporting that it was an issue and may be becoming more of a problem," he told reporters.

A recent study by the National Campaign to Prevent Teen and Unplanned Pregnancy found 22 percent of US teenage girls and 18 percent of teenage boys have sent messages or posted images or video online showing them nude or semi-nude.

"A student may be sending pictures to their boyfriend and they break up and he sends it to everybody," said Graff.

Aside from the disruption and trauma the practice can cause, Graff warns that the pictures potentially violate child pornography laws that ban sexually suggestive pictures of underage children.

"Any pictures of an underage child could potentially be child pornography and we are not really interested in seeing students punished criminally," he said.

"We want to put them on notice that it's just not something that really belongs in school."

(Agencies)