I am not sure if the girl I saw was wearing a diaper.
But when I went to school when I was about 14, I could see a girl one year older than me waiting with her class to do a photoshoot, a spoof one, and she was sucking on a pacifier had pigtails with ribbons and a short skirt, I was with my friends and dared not to stare, so I just glanced at her quickly so I am not entirely sure, she was standing quite wide-legged, but somehow I doubt that she was actually wearing a diaper, but the closest thing I have ever seen, very low on the diaper scene for me IRL.