Everything 10-year-old Mandy White says or does is a mistake.

In the opening scene of Bad Girls, a new play based on the children’s book by bestselling author Jacqueline Wilson, Mandy is being relentlessly bullied by three classmates.

So, when she calls her father “daddy” in front of them, it is a big mistake, the word hanging in the air incriminatingly. Her clothes - pink, and decorated with fluffy rabbits - are also a mistake. And the fact that her mother is 55 years old is a mistake. (The fact that her mother looks an elderly 70 is also a mistake, but one that lies with the production.) But then she meets Tanya, a spikey-haired older girl, who takes Mandy under her wing and defends her against the bullies. Though touchingly loyal to her younger friend, Tanya persistently steals clothes from local shops.

Eventually, this means that the two girls are forced to separate.

Many elements of Mandy’s bitter-sweet story are hauntingly familiar to anyone who has known the misery of puberty. The scenes of bullying are realistic enough to generate genuine discomfort. As Mandy, the astonishingly childlike 24-year-old Susan Harrison reflects the conflict of being too old for fluffy bunnies, yet too young to wear the sexy hairstyles and sparkly tops that Tanya suggests.

When she fantasises that she is a model, or that she has been adopted, her desperation is heartbreaking. And her devotion to Tanya is transparently grateful.

But many of the subtleties of pre-teen anxiety are lost on the pre-teens for whom the play is intended. Ten-year-old Jessica Dyke, from Morden primary, in south London, said: “I was a bit sad at the way Mandy was bullied. People should have helped her. But I wasn’t crying.”

Instead, the sniffles come from the adults in the audience, whose understanding is heightened by perspective. Lori Anderson, Morden deputy head, said: “We’ve all either been bullies or bullied. Adults have a distance kids don’t have, so we’re better able to relate to it.”

The play is on tour, details at www.badgirls.info