Pixie Lott, who performed at event honouring the campaign on Friday, shared an image of herself and her siblings posing for a school photo.
While Sharon Osbourne didn't share a photo of herself as a girl, she made sure that her two million followers saw a sweet snapshot of Kelly Osbourne as a youngster.
"Kelly as a school girl!" wrote 62-year-old Sharon wrote alongside the picture of her daughter, who looked distinctly different from her current appearance with her brown hair, red school jumper and matching tie.
Other stars who shared throwback photos included Downton Abbey actresses Laura Carmichael and Michelle Dockery, writer and journalist Caitlin Moran, former Prime Minister Gordon Brown and his wife Sarah, TOWIE's Lydia-Rose Bright and Bonnie Wright, who is best known for playing Ginny Weasley in the Harry Potter series.
Shakira also surprised her fans with her childhood photo, in which she looked virtually unrecognisable. Showing where her two-year-old son Milan gets his mop of brunette hair and large brown eyes from, the Colombia-born star posted a picture of her younger self in a cornflower blue pinafore dress.
The purpose of 'Up For School' is to highlight the number of girls who don't have access to an education.
"It's every child's right to experience the excitement of learning something new," reads a statement on the campaign's website. "But it's a right still being denied to more than 30 million girls all over the world. This International Women's Day, help make sure the whole world gets the message: girls education matters - and understands that one girl out of school is one too many. Nothing changes without pressure. Sign the #UpForSchool Petition. Act Now."