Brat tomboy idea: she's worn a hole in one of her shorts pockets so her hand ends up pushing further towards the fly when she's chilling hands in pockets. This looks like a crotch bulge to her, so she starts doing it intentionally to taunt the boys. "I'm bigger than yoooooou~"
Power move when they call bullshit? Unzip the fly with her other hand and stick the bird out. Little do they realize.