Framey got the gist of the thread.
Im absolutely no way in any form would I compromise my daughters confidence with regards to her period, of which she cannot help, and force her into a potentially humiliating sutuation.
At the very start of the thread, someone idiotically suggested it would be 'sending her a bad message'. 
Sure, telling an 11 year old to stop fretting about her WHOLE SCHOOL potentially seeing her walking away with a stained bottom is the best message to get to her. For fucks sake. Really?
Im future when she is on, Id make sure she has a uniform with pockets because it sounds rotten that its an issue for her each month.
But of course the PE kit is very different.
I had horrifically heavy periods as a child (and we need to remember she is just a child) and suffered at PE time the worst.
Luckily for me, I had a lovely mum like you that was very much in my side, and realised it was traumatic and that I didnt need a 'certain message' sent to me (again, seriously?! I feel sorry for that posters daughter - no doubt they have sons).