If you wouldn't say it in person, why say it online?
The National Crime Prevention Council in the USA has addressed cyberbullying in a number of short videos. They make the point very well: why behave differently online to how you would conduct yourself offline?
There's another, perhaps less obvious, message that comes across when you watch the videos. To quote from Edmund Burke ,
The only thing necessary for the triumph of evil is for good men to do nothing.
Use this as a starting point for discussion with pupils. Perhaps they could make their own cyberbullying video too: that approach has been used to great effect in a number of schools.