A girl in the year above my dd1 started in Y5, and all the junior girls knew about it, as she left the evidence lying about in the bathrooms for all to see. Dd1 and her friends were surprisingly mature about it (bearing in mind they were then Y4 themselves) and although it did spark quite a few very detailed questions, there wasn't any hysterical silliness over it.
Unbelievably, before that incident, there was no provision for girls who had started their periods. Now I believe there is, but I don't know what. I'll ask dd1 tonight. At our school the kids are allowed to bring bags in, so taking in a discreet package with pads wouldn't be a problem. I hadn't actually thought about that aspect of it (dd1 is quite a young Y5, and has the build of a stringbean, so probably not going to be an issue for a while). I'll ask her tonight what she thinks. But if her school banned them from bringing in bags, I would approach the headteacher directly and point out the problems this might cause. Chances are, they haven't even thought about it from that point of view. Alternatively, if they have to leave a PE kit bag at school, they could keep a little package in there for all eventualities.
I find it really hard to understand why the schools don't do a session for all children when they're in Y3 or Y4, so that everybody is at least equipped with the facts. And yes, birdsong, FWIW I think you should have the periods/pregnancy/sex conversation some time quite soon. You may be surprised to find that she already knows some of it, but there's a lot of misinformation floating around school playgrounds, so it's worth getting there first with some proper facts.