Nude in public, but no one notices

I have yet to find a story specifically exploring this, admittedly surreal situation. (Painted on clothing is closest, probably, but not the real thing, not what I'm talking about.) It is however one of my recurring fantasies, often reused for self-satisfaction. Well, this one is indeed out in the pure, absurd fantasy dreamland, but I would like to brainstorm how it could happen.

So, let say, a girl finds herself nude in school. Why is whole another (of course very relevant) matter, but she definitely shouldn't be. She likely (but not necessarily) is frightened and embarrassed, but somehow decide to just go forth, and let happen whatever will. For her utter surprise, nothing out of ordinary happen at all, it is just as she would be perfectly clad. In fact, teacher responsible for enforcing uniforms even compliment her about her flawless attire (probably she previously had problems with this). That only make her more nervous about what will happen when anyone will learn what must be in plain sight, that she is totally nude.

Now there are several ways this can go forth. The basic idea is that nobody see her nudity unless they know that beforehand, had been told explicitly, or find out in some other way. Then it is obvious, however

Let's say, eventually (maybe even next day) she had her sports period. She didn't change for that, as she decide (or must) stay nude, she just linger in the locker room for a while to not make it obvious. After however, she is forgetting about it lively chatting with her friend, and both girls go to shower without stopping by the lockers at all. One of them is stunned seeing her friend going under shower seemingly still in her sports attire and try to warn her against. As a result one or both have to admit they are is in fact nude, and had been for quite some time, but apparently nobody noticed.

And / or...

There is some obstruction at the classroom doors and she unintentionally brush her naked hip against crotch of a boy she have crush on. For her shock she unmistakably feels erect penis freely slapping against her, no cloth in between, while his hand touch her naked breast trying to stabilise them both. She goes few more steps away out of inertia, before looking back and see the boy is naked indeed. It is also quite obvious he now see her naked too. They eyes meet, and they signal 'hush' to each other simultaneously.

Well, that's the effects. But wtf is happening here?

There may or may not be need to explain everything, but it is necessary to be at least able to speculate about causes of this sort of mass madness.

Is it some massive conspiracy? But how they control the uninvolved?
Mass hypnosis?
Hard magic of some kind?
Mind controlling nanites or alien invasion?
Perception or even reality bending psychic powers, conscious or not, that probanly work only while the person is naked?
Dimension shifting or mixing of parallel realities (the other being all nude as normal)?
Is everyone in fact nude, and only pretending?

Also, how long the lone or few nudies can keep their secret, and how far they can push indecent behaviour hiding in plain sight?

Cheryl had never been more despondent. She'd been wandering the halls and dining room of Lacknookie High School for three hours and nobody - not one single person of either sex - had noticed that she was dressed in nothing more than sandals and perfume.

What the fuck was wrong with these people? she wondered.

Popping into the girls' room, she spun around in front of a mirror. Boobs - check. Pussy - check. Bum - check. All the standard equipment in the standard places. The testicular-equipped half of the school (and a few others she'd sometimes wondered about) - were they all fucking dead?


Take it from there...

This might be an invisibility story.

Maybe it's a sort of hypnosis.

Maybe one has to be naked to see her naked.

Maybe it's a piece of jewellery that causes people to act as if she had clothes.