"It got worse and worse and carried on in and outside school," said one of the 14-year-olds who was involved. "We got angry and upset and we just wanted it to stop. The fear of it all made me want to stop coming to school."
Mark Tilling, the deputy head in charge of pupil wellbeing, said: "There was physical and verbal bullying, but the online aspect was the worst. I had not come across it before being used to that extent."
The two groups of four girls were using instant messaging services, social networking sites and text messages to intimidate each other.
Often the person posting the comments remained anonymous, increasing the sense of fear.
"Everything was said over computers," said one pupil. "When it went on some sites we didn't know who it had come from. It made you worried when you left home."
Val MacFarlane, regional co-ordinator with the Anti-Bullying Alliance in the North East, said cyberbullies often take their threats further than they would if they were dealing with their victims face-to-face because they cannot see the effects they have.
Teachers at the school called in parents in an attempt to resolve the problem, but realised they could not do it on their own.
They called in experts from the Redcar and Cleveland anti-bullying team to run a series of sessions with the pupils involved. The girls were eventually brought together to talk about why they were doing it. Their parents agreed to ban the pupils from using computers at home.
"We had to get everyone together and build relationships," said Mr Tilling. "They made such good progress they went on a bowling trip together as a reward. They are not the best of friends but they are civil and the bullying has stopped."
The school is planning a peer-mentoring scheme to give younger children an older pupil they can turn to. The girls involved in last year's bullying will be among the first to take on the new roles. "They were very keen to get involved," said Mr Tilling.