OP, I know you're freaking out and not ready. You may just be surprised by your dd, even though she may not be mentally ready. In all honesty, preparation goes a long way in helping your dd to become "ready mentally".
I like Beth's suggestion of not necessarily wearing a bra but more of camisoles with build in tanks. I went to an all girls school, and was one of early developers. I had my first period at 9.5, and 2nd girl in my class to get it. It wasn't the actual period that traumatized me , but huge lack of preparation. My mum avoided having the "talk" for so long that it kind of became too late for me. I would have really appreciated just having a pad in my school bag, as I had it during my swim lesson at school. Talk about social suicide! So if I knew I had a pad in my bag, it would have saved me the embarrassment of asking my teacher who luckily happened to be a female.
That was a bit traumatic for me. So I think if your dd felt prepared and the like, it would be a non issue for her. Also, other pp said the late bloomers had it harder mentally vs. Early bloomers. Since I went to all girls environment, I find that to be quite true. I had a few who didn't get their periods until 15 years old and boy, the worries, drama etc they had till it happened for them.
Be prepared for the other side of the coin; late. Just because you had yours in 5th grade isn't a guarantee she'll be "on time".
Mummy to DS1-6/11 and DS2-1/14