Maybe it's because I'm a man that I found this disturbing when I first learned of it—after all, I never had to deal with my own breasts growing, and the ensuing change in how I was regarded by my peers (actually, my breasts did grow, but I digress). A friend, a year younger than I, says she had one of these growing up.
For actual women, I imagine, it was a way to experience the phenomenon at a remove, as it were. I suppose (again, never having been a prepubescent girl) geeklizzard isn't wrong to say that menstruation is more disruptive, if nothing else, than bigger tits, but the visible bodily changes can be traumatic too.
Someone I know has told me the story of being called into the principal's office to be queried about her home life. Her attire, her teachers felt, was a little too mature for a junior high school student. Now, the outfit was more modest than what at least some of her schoolmates were wearing that very day. It was her body that was more mature. The faculty were projecting their own perceptions onto her nubile young body, the sick freaks.
If she'd had Growing Up Skipper (long off the market by the time she was old enough to appreciate it), or perhaps if her teachers did, the whole incident could have been avoided.