I've had a few over the years.
The first was in 1st grade. There was a girl in our class who was a special ed kid, but would still be in our classroom most of the time. One time I noticed what had to be a diaper or pullup sticking above her waist band.
The second was a similar scenario in 5th grade. They had this program where we could use our recesses time on some days to go help/play with the special ed classroom. These kids were usually pretty severely impacted by whatever it was that required them to be in a separate space most of the day. I vaguely remember noticing that some of them (but not all) wore diapers. One time one of them just started wetting his pants in front of the school library, and he then whipped his dick out and started peeing all over the floor while jumping around like he was hanging on to the end of a fire hose. (Im assuming he was trying to avoid getting his pants wet?)
Third time was in a program where we work with kids on the spectrum by doing performing arts related stuff with them, and I noticed at least two of the kids I would interact with wore diapers or pullups.
Another time I was in a restaurant and went to the bathroom. It was one of the ones where only one person is in it at a time, and it was occupied when I got there. A minute or so later the door opens and a kid in a wheel chair rolls out. I go in and use the bathroom and all that, but when I go to throw away the paper towel I dried my hands with, I saw a diaper in the trash can. Upon closer examination, it was very warm, and it was definitely sized for an older kid, not a young child/baby. I assumed it was from the kid in the wheelchair.