Headteachers are becoming increasingly concerned by attacks and campaigns against their staff made by parents on social networking websites.
According to heads' union the NAHT, the fastest-growing source of calls to their advice line is members seeking guidance on how to deal with abuse against teachers and "unsubstantiated allegations about management decisions" via sites like Facebook and Twitter.
The union has been forced to publish guidance to help heads deal more effectively with misuse of social networking websites.
Because users are afforded near-total anonymity, gossip and rumours can easily grow into petitions and campaigns, the NAHT said.
The union has also called on the likes of Facebook to crack down on abuse far more promptly.
NAHT general secretary Russell Hobby said: "Parents have a right to express their views and complaints should be heard; schools can only benefit from constructive feedback.
"Too often, though, social networking sites are a medium for the unreasonable and the unprincipled, and have a momentum out of all proportion to reality.
"The lack of accountability or moderation can whip up a cycle of abuse and sustained intimidation, causing immense harm."