Girls Aloud played the school band during a brief scene at the end of the first St Trinian's picture, but Sarah will play a considerably larger role in the new film.
The production schedule lists her to be available for 30 days to shoot her part playing Roxie, a new girl at the school.
'Roxie's an indie chick, a very independently-minded girl who has been through several schools.
'She's tough, cynical and you don't mess with her,' said Barnaby Thompson, who's directing the film with Oliver Parker.
Thompson and Parker cast Sarah because 'we liked her vibe'.
He added: 'Sarah's a real rock 'n' roll chick on and off the screen, she's fun.'
The returning cast includes Rupert Everett, who will reprise his role as unconventional headmistress Camilla Fritton, Colin Firth, Gemma Arterton, Talulah Riley, Jodie Whittaker, Juno Temple (pictured), Tamsin Egerton, Celia Imrie and Fenella Wollgar, alongside newcomers to the franchise Ella Smith, Zawe Aston and Montserrat Lombard.
The film see the girls embark on a rollercoaster-style treasure hunt for the legendary Fritton’s Gold, which sees them face the villainous Pomfrey, played by Tennant, and his sidekicks from the women-hating secret society known as AD1.