This sort of physical violence is rare among girls. They are much more likely to resort to more covert types of bullying, such as nasty looks, catty comments or excluding behaviour. As such, it is hard to measure the extent of bullying by girls because it is not always easy for teachers, parents or even the victim to recognise.
But this incident is all too visible. "The girls are acting in a group and that is where their strength comes from," says Professor Helen Cowie, who has written extensively about bullying and violence in schools. "Although the ringleader will typically feel little empathy for the suffering of peers, the other members of the gang will have varying degrees of remorse and guilt."
Play on that discomfort, advises Rita Adair, an educational psychologist with the Anti-Bullying Alliance. "Give the bystanders the skills to know how to respond and stop the violence in a safe way," she says.
It is possible to change the ringleader's actions by empowering those around her. The chief bully, or "Queen Bee", as Rosalind Wiseman calls her in Queen Bees amp; Wannabes, needs the laughter and attention of the crowd. If the hangers-on refuse to condone her behaviour, she is left isolated.
Discuss different levels of culpability with the class. Who is really to blame for this: the main bully, the sidekicks, those who fan the flames or the seemingly innocent bystanders? The class will realise bullying is everyone's responsibility.
Dr Val Besag, the author of Challenging Girls, advises teachers use the film Mean Girls, starring Lindsay Lohan, to illustrate how pupils can be manipulated by friendship groups. "Girl bullying is nearly always done by friends," she says. "It can easily be denied with a, `Oh, it's only a game' comment, which makes it even harder to deal with."
After whole-class exercises, Dr Besag recommends working with individual bullies and other ringleaders. A frank discussion about what is going on and what can be done to stop it may be enough.
"Give every child the chance to change," says Dr Besag. "Explore with them what bullying is, the effect it has on the individual and why they do it." However, if there are no signs of improvement, exclusion or other sanctions may be required.
Burgess Hill School for Girls in West Sussex rarely has to deal with bullying. If pupils do have a problem, they can put a note in a sealed box or send a message to a secure email account. Four sixth formers are responsible for allocating the issues to trained Year 10 and 11 "stars", who listen to the problems and make suggestions.
Liz Laybourn, the deputy head, receives an anonymous summary of each case so she can trace and record which pupil "client codes" keep re- occurring.
"Often the bullied girls just need a point of contact to offload," she says.
More serious issues are referred directly to Mrs Laybourn or another member of staff. She first reassures the bullied girl before sometimes offering help from an external counsellor. She then talks with the bullies to try and come up with a solution.
But Professor Cowie says many bullies display signs of various mental health disorders and may need help, rather than sanctions, to overcome violent tendencies.
Next week: Consistent lateness.