A survey suggests pupils, and parents, are using these networks to insult school staff, make allegations of inappropriate behaviour and to comment on performance in the classroom.
In one case, a teacher reported they had been racially abused, while another said comments had been made about their sexuality.
But many teachers are afraid to report online abuse, often because they did not think anything could be done, or they did not think it would be taken seriously, the survey suggests.
The poll, conducted by the NASUWT (National Association of Schoolmasters Union of Women Teachers) union, questioned more than 7500 teachers about the use of technology.
It found 21% said negative comments had been posted about them on social media sites.
Of these, almost 64% said the comments had been made by pupils, 27% said they had been posted by parents and the rest said these had been made by both pupils and parents.
Just over three in five, 61%, said the pupils posting the comments had been aged between 14 and 16, with one-third (35%) saying these youngsters were between 11 and 14. Around one-fifth (21%) said those responsible were 16 to 19, while 3% said they were between seven and 11.
Of those subjected to offensive posts made by pupils, the most common type of abuse was comments about their teaching -followed by insulting comments, or videos and photos taken without their consent. Around 4% had faced allegations of inappropriate behaviour, and the same proportion had experienced threats.
The most common types of abuse by parents were also comments about their performance in the classroom and insulting remarks.
One teacher said a parent had told them they were "a nasty teacher, telling me to go 'home".
Another said: "I had a video taken from the corridor while discussing a report card with another pupil. I was accused of calling the pupil a bitch when I did not". A third told researchers they had faced a false allegation that they had punched a pupil.
More than half (58%) of those surveyed did not report abuse from pupils, with 64% saying this was because they did not think anything could be done about it. Of those that reported comments made by a pupil to a headteacher, around 40% said no action was taken.
General secretary Chris Keates, said technology had transformed the lives of teachers and pupils, but steps need to be taken to protect teachers from abuse. "Teachers are often devastated by the vile nature of the abuse they are suffering," she said.