Police were called into a Worcestershire school after a pupil posted " offensive material" on the internet about another child.
The abusive comments were about a teenage girl after an argument with another pupil at St John's Middle School in Bromsgrove.
Both the police, the school and education authority refused to comment on the nature of the site which has now been closed.
An anti-bullying leader claimed the incident highlighted a growing trend which sees bullies exploiting modern technology to intimidate their victims.
St John's head teacher David Macey said the perpetrator had been disciplined.
"We followed our antibullying code to the letter. It happened during the holiday and appropriate action has been taken and the matter has been dealt with," he said.
"It was offensive material. This was a very unusual incident and I am delighted to say the advice we have been given has enabled us to deal with it."
A parent of a child at the school, which caters for nine to 13-year-olds, said: "Police were called in because someone created a website that was offensive to another pupil.
"It was posted from the school or created at the school and got on the net through the school."
A spokeswoman for West Mercia Constabulary said: "Basically it was one teenage girl making nasty comments about another on a private website.
"We are investigating and the website has been frozen in the meantime. We were contacted by one of the girl's parents and made inquiries at the school."
Education chiefs in Worcestershire said antibullying guidelines issued to schools have recently been upgraded to take account of more modern methods of intimidation.