I’ve been wearing a bra since before I was 9. I was a C-cup at 10.
If your daughter needs to wear a bra, as I did, get her a damn bra. She’s probably in secondary/high school, and getting changed for physical education means she’s exposing her bare breasts to her classmates. Don’t put her through that.
If she is a late developer with no breasts yet, she’s still getting changed for PE in front of other girls who are probably making her life miserable by mocking her for her lack of breasts. Don’t put her through that either. There are plenty of bralettes and training bras she can wear for now.