@zerio I've read all of Onio in a sweep right now, but wasn't it in this one he mentioned how the uniform rules were a bit lax? Or was that in one of the other two?
Anyway, if it's slightly lax then she could get away with it as a tomboy (especially since school is over, maybe she just removes it as soon as class ends just because she doesn't like it very much, especially in the heat when she wants to button up a bit.)
But I don't think it's a major change, just a slight twist. But, if you want to disbelieve it might just be that big judo friend decided to somewhat switch camps.