As many as 50 000 German teachers could be victims of cyber-bullying, on internet video-sharing sites, via email or simply through their mobile phones, according to a union survey.

Eight percent of 500 teachers quizzed by the country's Education and Science Union said they had either been victims of online harassment or knew colleagues who had suffered from the modern phenomenon.

The union, which said its sample was sufficiently large to extrapolate national estimates, indicated that 70 percent of attacks emanated from pupils. The threat appeared to be the same for male and female teachers.

"Many teachers feel as if they are under constant harassment," said Marianne Demmer, the union's vice-president. The union is calling for a debate on potential measures to protect its members from a "widespread menace".

Examples uncovered by the survey included one in Bavaria in which a student put a video montage online showing a fictional execution, with one of his teachers depicted as the victim.

Another was the victim of an internet whispering campaign which said he was a paedophile, with a female teacher also subjected to a series of telephone calls amounting to sexual harassment.

While the teacher who fell from her chair after pupils cut its legs also became an online video-sharing star, consequences have proved serious, the union said, citing depression, insomnia, requests for transfers and early-retirement applications.

Most victims alerted head teachers or local police, but many felt there was nothing they could do about the issue.

AFP