Mischa Barton picks her way through the battered old school building in Henley that serves as the set for the return of the St Trinian’s franchise. This nostalgic realm of jolly hockey sticks and high school japery has been reimagined for the 21st century by Ealing Studios.

The film’s directors, Barnaby Thompson, who acquired the studios seven years ago, and Oliver Parker, who directed the first Ealing movie in almost 50 years with The Importance of Being Earnest in 2000, have been keen to bolster the star power with names including Rupert Everett, Colin Firth, Russell Brand, Stephen Fry and Girls Aloud. Barton plays J. J. French, a former head girl who has moved to the US and prospered in the media world, before returning to St Trinian’s to help the girls in their bid to save the school. “The idea is that J. J. went into PR and became very sassy and quite ruthless,” she says. “She gives the girls some advice, and it turns into a bit of a treatise on pop culture as she decides to teach them all the tricks of her business.

“It is supposed to be rife with funny clichés. I don’t want to give too much away, but let’s just say that it hones in on the superficiality of the business, and all the silliness. I was considering playing one of the main characters in the film. But it didn’t work out and Barnaby wrote the cameo for me,” Barton says.

“I was originally talking to Barnaby about playing Annabelle, the lead girl, but then this is a very English film,” adds the 21-year-old actress.

“My family’s English, I have seen the films, and I thought that they meant a lot to a lot of people, so it just seemed more appropriate to come in and do a cameo role, rather than play an established British character that people really love.”

She may speak with an American accent, but Barton was born in the UK to an Irish mother and English father. She has fond memories of growing up in London, moving to New York with her parents and two sisters, Hania and Zoë, when she was six years old. Yesterday, she donned a St Trinian’s uniform for a photo-shoot and, she chuckles, it took her back to her childhood.

After breaking into theatre when she was 9 – co-starring in the off-Broadway production of Tony Kushner’s Slavs! – Barton is inured to the silliness and superficiality of show-business. And as soon as she was cast as the troubled belle Marissa Cooper in The OC, which ran from 2003 until this year, Barton became a pop-cul-ture icon and tabloid favourite.

“After we finished the first season of The OC I started running into paparazzi all over the place,” she laughs. “And I became popular with the tabloids, so I knew straight away that I’d need to make a few changes in my life.”

These involve keeping a lower profile when she and her friends venture out for the evening. “The media is so quick to pick up on things, so you can’t go around dancing on tables and drawing lots of attention to yourself. If you do anything, they will try and paint as you this wild party girl. That really isn’t me.”

Did she have a St Trinian’s-style schooling? “When I was at school I wasn’t out all the time. Maybe I wasn’t quite head girl material, but I worked hard. Because I was working as an actress, there was always that sense of a normal schooling being taken away from you. Since then I’ve always kept things very focused on my family and my career.”

That career has spanned more than half her life. She was offered modelling work when she was only 11 years old, shooting a campaign for Calvin Klein jeans, she made her film debut in Lawn Dogs (1997), and appeared in The Sixth Sense and Notting Hillin her early teens. It was The OC, however, that propelled her to stardom. Following her departure after the third season last year, the show’s ratings tumbled; it ran for only one more season.

“Of course I’m grateful to The OC,” she says. “It has opened up a lot for me. Ever since I did that first off-Broadway play I knew that acting is what I wanted to do with my life.”

She’s happy to deal with the media pressure. “You read about all these different guys that you are supposed to be dating, while really I’ve only had a couple of boyfriends in my entire life.”

Last year the gossip press tracked Barton’s romance with rocker Cisco Adler from the band Whitestarr. Before that she dated Brandon Davis, the oil heir and former MTV star. She will not be drawn on any current romances, but does admit that she is always “very careful”.

“I’m not one of those girls who likes bad boys who are going to treat them badly, and I’m also not sure about dating another actor. I can’t remember meeting any actor and thinking that I must get him to take an interest in me. I also have to spend far too long looking in the mirror, so it wouldn’t be a good idea to share my life with a man who has to care about his looks, too!”

Anyway, she says she has been too busy of late. She has recently finished filming the dramas You and I (Finding tATu), Don’t Fade Away, and the comedy Assassination of a High School President, in which she stars alongside Bruce Willis. Her next project, Malice in Sunderland, is, as the title suggests, set in the UK.

“It’s been great coming back to England,” she says. “I still have family here. My dad’s with me today on the St Trinian’s set and I’m sure people will like the new version.”

And she has a good idea what it is about the films that people like: “Looking at the older girls, they are really sexy,” she says smiling naughtily. “I was watching them strutting around, and we all know how boys feel about girls in school uniforms.”

St Trinian’s is out on Friday

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