I wore diapers in Kindergarten and I wore the thick plastic backed Pampers diapers.
You could see the diaper bulge in my sweatpants and it got bigger when I peed in the diaper.
The diaper waistband was also noticeable.
I also wore an adult diaper in school when I was around 12 which I think was an attends diaper that crinkled loudly and I would let the waistband of the diaper stick out of my pants and I wore a short shirt so that it would allow the diaper to be seen.