According to Russell Hobby, the NAHT general secretary, social networks have been used to make personal attacks on staff as well as "unsubstantiated" allegations regarding management decisions.
"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," said Hobby.
"The lack of accountability or moderation can whip up a cycle of abuse and sustained intimidation, causing immense harm."
The NAHT is calling for Facebook and other social networks to speed up the time it takes to act on reports of abuse, while also improving the way such abuse is reported.
Facebook said it has "clear rules about content which can be posted on the site", and provides people with "robust mechanisms to report content or activity that breaches our terms".
"Facebook has worked hard to develop these reporting mechanisms, but the reality is that many discussions that take place on Facebook reflect those that are happening offline," a spokeswoman for the social network added.
"However, while you can't report a conversation outside the school gates or easily stop a person sending abusive, anonymous emails, Facebook gives people the tools to report offensive content they are concerned about."