I started my periods when I was 9, as did my daughter. I remember being absolutely terrified, because I had no idea what was happening. I remember doing everything I could to skip PE at primary school, because my bulky pad showed through my gym knickers. After a few months, one kindly teacher noticed what was happening and suggested to my Mum that I should be allowed to wear shorts.
How things have changed! When my daughter started her periods, she was confident enough to tell me, so I rushed out to the supermarket to buy some lightweight pads. I rang up the school next day. The school had procedures in place for girls who started early. My daughter was allowed to change in a toilet cubicle rather than the classroom and she was told to have a quiet word with the teacher if she needed to leave the classroom to change her pad or if she ever felt too embarrassed to do PE. As far as I know, she never did skip PE and never had to deal with the embarrassment of having a red stain on the back of her skirt.