1) At a school camp in 7th grade, for some reason a bunch of girls went into the showers without taking clean clothes with them, and could only get back to their cabins from the showers with a towel wrapped around them. I remember seeing them looking all embarassed at having to make the run to their cabins wearing only a towel, but back then I didn't really understand why they were so embarassed. Towels were enough to cover. I found it funny they were needlessly embarassed.
Now that I think about it, I think the boys got back early from some camping activity, and the girls didn't expect us to be there, hence why they were in the showers with no clean clothes.
2) as a joke, a girl lifted up the dress of her friend when she was talking to me and some other boys. She crept up behind her friend and lifted it high so we could see her panties, bare legs, etc. why I remember this is because the girl went completely red out of embarrassment, lol.
3) this one I don't think the girls were embarassed, I don't even think they knew, but I'll mention it. We had a drill in class where everyone had to get under their desk. Two girls were sitting directly in front of me and my friend, and the way they were hunched over in their short skirts meant we could directly see up them, seeing their panties. My friend and I were trying to hide our laughter the whole time.
If you enjoy these, I'll try to think of more.