The mother of Keane Wallis Bennett said close friends of her daughter will wear the outfits as Keane had "loved them".
The 12-year-old died on April 1 when a free-standing modesty wall at Liberton High School in Edinburgh fell on her while she was changing for her PE class.
Her death certificate revealed that she had died from "multiple injuries" when the wall collapsed.
Keane's funeral will take place tomorrow with her family encouraging people to wear "bright" colours to the service at Mortonhall Crematorium.
Keane's mother, Abbie Wallis, said her friends will wear onesies to celebrate her daughter's life.
Writing on social media, Ms Wallis insisted there was no dress code, but said some friends would be wearing the outfits, while she would wear something "bright and airy."
Ms Wallis said: "I know some of the children are opting for onsies (sic) which is great as Keane loved them. I'm gonna pass on that though and go for something a little bright and airy."
Keane's funeral come as councillors in Edinburgh have revealed the gym building where she died could be demolished following the completion of Police Scotland's investigation.