Teachers have called for the expulsion of pupils who insult staff on internet sites such as Facebook and Bebo after a popular girls' school became embroiled in a cyberbullying row.

Nearly 30 pupils were suspended from Grey Coat Hospital School, a Church of England secondary in central London, after "deeply insulting comments" about a teacher were posted on the networking site Facebook.

Teaching unions have reported an increase in the number of cases coming to their attention, with one in five member of staff experiencing problems.

Mary Bousted, the general secretary of the Association of Teachers and Lecturers, said pupils should face expulsion in serious cases.

"In cyberspace, hundreds of people can laugh at a teacher or child's humiliation," she said. "Some of these incidents can be really, really nasty. Young girls, in particular, can be very malicious. Teachers attacked in this way have no chance to defend themselves. Their professional and personal reputation can be left in tatters."

Dozens of Gray Coat girls signed up to a Facebook member group named The Hate Society, which contained messages attacking the teacher. Their comments are thought to have come to light when a pupil printed off the web page and passed it around the school.

Parents of the girls involved were telephoned at home by staff at the end of last term to inform them of the incident. The female teacher who was the target of the abuse is understood to have received counselling.

Rachel Allard, the head teacher of the school which is heavily oversubscribed and was rated "outstanding" in its last Ofsed inspection said the vast majority of parents who had been to see her about the incident were supportive and understood why she had taken firm disciplinary action.

But some of the pupils who received temporary exclusions have claimed that the punishment was too harsh.

"I feel completely ashamed and disgusted to go to the school," one told this newspaper. "They (the school) took it too far by publicly telling people about it.

"There were many girls who got suspended and we feel as students it is our right to express our own opinions. The school handled it very badly by suspending a huge amount of people. They have been unreasonable and we want justice.

"It was just a stupid game that no one took any notice of. We have said sorry and understand that what we did was wrong, but yet they continue to carry this on."

Another girl described the abusive website as "a bit of harmless fun".

While The Hate Society page has been removed from Facebook, disparaging comments about the teacher remain posted on another website.

One pupil says: "She treats students like dirt. She actually sniffs out trouble and just has a pure hatred of children."

A former student writes: "She seems to go out of her way to make people feel stupid, upset, inferior and generally miserable. To be honest, she is one of the only things about the school that I really do not miss."

Another says: "Awful, awful person. I shall not refer to her as a teacher because she is not."

Cyberbullying of teachers is becoming an increasing problem, with children attacking teachers on website, posting videos of them on YouTube and taking picture of them on mobile phones to pass around.

In a recent survey of teachers, most said their schools did not have disciplinary codes to deal with cyberbullying, and where they did exist they were not properly enforced.