Harriet Walker: Nothing is as toxic as the venom one teenage girl can inflict on another

Channel 4's Educating Essex is a brilliantly appointed snapshot into life at a secondary school through academic trials and tribulations, workaday acts of heroism and the gym kit tyranny that teenagers inflict on each other.

This week's episode stalked the Popular Girls and catalogued a specifically adolescent sort of intifada.

Initially inseparable thanks to a mutual loathing of Pi, the Hot One stopped speaking to the Bright One without giving any reason, and took most of the female student body with her. The Bright One tried to get on with lessons, every so often asking why communication had ceased as suddenly as someone cutting a phone line. They smirked and failed to answer; she took three days off.

It was sickeningly familiar. The agony of not knowing what you are supposed to have done; the desperate need for reassurance, but meeting only impassive, unconcerned faces – or worse, being looked through and spoken over. You're a wraith; you become hollow, a walking outline to be filled with other people's opinions of you because you have no perspective of your own. It is an all-consuming agony. Howls of impotent rage and sympathy ripped through Twitter. "Why are girls so cruel?" some asked. "Thank God school's not forever," counselled others. Magazine editors, PR gurus, politicians, celebrity chefs: all felt her pain personally and keenly. There is nothing as toxic as the venom of teenage girls; it burns most corrosively when dripped from glossed pouting lips.

Why do girls treat each other like this? Why so insidious and spiteful, why so hell-bent on humiliation? It lies in an innate sense of competition and insecurity that can't be blamed on nascent gender relations, because it is by all accounts worse at girls'-only schools. It's depressing to think this barbarism is a phase women go through before realising the need to stick together. Not all of them do it, of course, but with smartphones and social media, this sort of behaviour is actually gathering strength. I hope the programme is enough for schoolgirl phantoms to take succour and focus on getting out with the grades. But when you're the centre-point in a circle of people who don't want you, it's hard to concentrate on Pi.