Teachers are facing death threats, abuse and allegations of serious crimes by pupils and parents through social networking sites, a poll suggests.
One was even told her throat would be slit, while another was accused of being a rapist.
Online abuse of teachers is widespread, with more than two-fifths (42%) of those questioned saying they had been a victim of it, a survey by the NASUWT teaching union has found.
Of those, more than three-fifths (61.2%) said a pupil had written an insulting comment about them on a social networking or internet site, while more than a third (38.1%) said a student had made comments about their competence or performance as a teacher.
And nearly one in 10 (9.1%) said they had faced allegations that they behaved inappropriately with pupils.
One teacher told the survey that a student had posted that they were going to "slit my throat", while a second said a pupil had written "my English teacher should actually die" and a third said a student had posted that a teacher "is a rapist".
Another teacher had faced claims that they were unable to do their job and should be sacked. The teacher told the survey there had been "lies about me: saying inappropriate things to pupils, insulting pupils, having sexually absurd behaviours. Generally defaming and insulting me, spreading rumours about me throughout the school."
In some of these cases, action was taken against those responsible.
The poll, which questioned 1,522 teachers, found that the majority of pupils responsible (60%) were aged between 11 and 16, although some teachers reported comments by primary schoolchildren.
And it is not just pupils who are responsible for posting abuse about school staff online.
Around 16% of those surveyed said that they had had a comment posted about them by a parent in the last two years.
Of these, more than half (52.7%) said they had faced insulting comments, 47.7% said there had been comments about their performance, and 13.3% said there had been allegations of inappropriate behaviour towards students.
The poll reveals that nearly two-thirds (64.1%) of those questioned said they had reported an incident of online abuse, with teachers most likely to tell their school's headteacher or principal.
Among teachers who did not report an incident, more than half (53.2%) said they did not think anything could be done.
And of those who did tell someone about online abuse, just a third (32.2%) said they thought the pupil had been disciplined appropriately and they felt supported.
NASUWT general secretary Chris Keates said: "Some of the findings in this survey are truly shocking.
"Yet there are no adequate procedures in place, locally or nationally, to protect teachers.
"It is clear that some employers are seriously failing in their duty of care by neither having appropriate policies in place nor taking incidents seriously when reported."
The poll also asked teachers to say which social networking or other internet sites comments had been posted on. Some 751 people responded to the question, with Facebook the most commonly cited (by 77.1%).
Facebook said that it has "clear, strong rules" governing behaviour on the site, with tools on almost every page allowing anyone to report inappropriate behaviour. This is then looked at, and content or pages can be removed.
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