Harding, right, will appear next week in BBC Two drama Freefall
Doctor Who star David Tennant and Girls Aloud singer Sarah Harding will join the cast of the new St Trinian's film.
The pair will join Colin Firth and Rupert Everett - who both starred in 2007's St Trinian's - in follow-up The Legend of Fritton's Gold.
Tennant, 38, will play villain Pomfrey in the latest instalment of the rejuvenated franchise.
Harding, 27, who made a cameo in St Trinian's with her bandmates, will play "a cool new schoolgirl".
Ashes to Ashes actress Montserrat Lombard will also appear.
The original 1950s St Trinian's films - including The Belles of St Trinian's and Blue Murder at St Trinian's - featured stars including Joan Sims and Barbara Windsor as naughty schoolgirls.
The 2007 reworking, which also starred Gemma Arterton and Russell Brand, breathed new life into the franchise, and took £15m at the UK box office.