Naked under graduation gown
Unless your gown is more like a priest's frock then I can't see how you could. Certainly here in the UK, mine was an open fronted gown worn over the top of a suit. Even if it were closed at the front, you'd still need trousers unless it were right down to the floor, and a shirt/tie unless you button it tight up to your neck
I've heard of girls doing it on a dare, but maybe that was just that 70s movie somebody mentioned.
I don't think my graduation ceremony from high school was cap and gown. And I didn't even go to my college graduation, since I didn't see the point of the ceremony. I just got the diploma later.
Would I have gone naked under a gown? Probably not. I wasn't that adventurous then.