Reading that reminded me of a similar story I experienced myself

Unlike my current self, when I was little kid, I wasn't any pervert but usual well-behaved shota-kun. In some elementary school summer, I was sent to a summer "camp" school where we had swimming lessons. Before and after swimming lessons, USUALLY first girls changed and later the boys. Changing involved seeing each other fully naked, and as I was still extremely socially awkward (thus scared of being seeing naked too), I would either get in early and use the single cabinet or wait until everyone left and use the cabinet (since kind of everyone wanted to use it to some degree). There was a sympathetic teacher who would turn a blind eye to my general avoidance of such group events.

One of the lessons, the "older" grade's girls (but must be still liek middle school at most) entered the changing room while I was changing in the cabinet! I was too scared to warn them at first, but they eventually noticed the single changing cabinet was occupied. I was too scared at the time to talk beyond telling them I was changing (they asked if I was in their grade, I didn't know what to respond). They didn't initially make a big fuss, just asked the teacher to change their clothes after the weird guy who refuses to share his identity get out or sumthing... but teacher-san probably guessed I didn't want to be seen so he ordered the girls to change while I was in the cabinet... They freaked out but obliged

So I was too young to understand the situation then but there was a point in my life where I was alone, naked, with few tens of nee-chan aged schoolgirls, who were also changing naked from (or to) their mizugi sugata. After maybe an hour or two of waiting in the cabinet, after making sure noone was around anymore, I sneaked out of the changing room

Thank you sensei. I sometimes reminisce about that time while fapping to mizugi ero.