There is a growing trend for children to leave offensive comments on the sites or to make allegations against staff on social networking sites, National Association of Schoolmasters Union of Women Teachers (NASUWT) head Chris Keates said.

He said the rise in abusive messages has had an impact on some teachers’ health, while others say the public criticism has damaged their careers.

"They actively encourage the abuse of school staff," Keates said. "Teachers are named, exposed to ridicule and subjected to false and malicious allegations."

The union wants the government to review the law to give teachers more protection from being named on sites such as www.RateMyTeachers.co.uk.

Nearly a million teachers at 7,500 schools are listed on the site, with staff given an "overall quality" rating based on their popularity, clarity and helpfulness.

While most comments are positive, some random negative views include: "An irritating harlot"; "She should be locked up in Belmarsh (prison)" and "Nice bloke, can’t teach."

The site’s founder Michael Husey said 70 percent of the comments were positive and teachers find the feedback useful.

All postings are read before being published. Any that break the site’s rules, which include restrictions on bad language, appearance or sexuality, are deleted.

"I know it helps teachers," he said. "We get emails back all the time from teachers … who are using the Web site to bring them closer to their students and create mutual respect.

"They (unions) are attacking things that undermine their power structure. They aren’t adjusting too well to the information age."

Children and Schools Secretary Ed Balls said he understood teachers’ concerns about online bullying.

"Clearly anything that undermines the authority of teachers has to be stopped," he said in a statement.

Via: Reuters