girls are now at risk of bullying 24 hours a day through text messaging, Facebook and Twitter, a communications expert has warned.
Children's advocate Jean Gross said social media and texting have given girls more ways of excluding each other.
She said she was concerned that such behaviour was too often dismissed because of the view that "girls will be girls and will always be mean to each other".
Mrs Gross, a communications champion in the children's sector in England, added: "Anybody who's been a girl knows that girls can often make each other's lives a misery in school through communication, through this business of forming of cliques and of being one minute 'you're my best friend' and the next day, suddenly, 'you're not my best friend, I've got another best friend'.
"Now girls have got even more ways of excluding each other through communication, so they used to do it just through verbal language, now they can do it through texting and de-friending and all of those things."
Mrs Gross said she believes that girls know when they are being mean to each other, but added: "I don't think they know quite how much it hurts, because the people who may be doing it, it may not happen to them, they're protected..."
She added: "Teachers can teach some of the skills of say, not following blindly, you can teach assertiveness, you can teach girls about this kind of bullying, teach them to understand it."
Mrs Gross also suggested that girls could read and discuss Margaret Atwood's book Cat's Eye, or the Hollywood film Mean Girls, starring Lindsay Lohan, both of which deal with girls excluding each other.
Mrs Gross warned that technology has given the girls the chance to be cruel to each other "24-seven".