@LupusDei"Invisible" unexpected public nudity.
Okay, that's a weird kink of mine, but let's consider, a mind controller or reality editor could have his own similar, and make someone to walk around naked and everyone to accept it in stark contradiction to usual expectations, for own titillation or whatever other purposes they might have. Meanwhile the main character seems the only one, or one of very few aware of that dissonance. The main character may or may not be the nude person.
Let's say... in the school pool (sorry, school is just such a convenient ready setup for such) our point of view character seems to see, and after much doubt has to conclude, that one of the girls (Nancy) is indeed fully nude. No one else seems to have a problem with it, like they wouldn't notice or mind at all. He (or she) may maneuver to her vicinity and politely note her condition, just in case she herself may not be aware, and nobody else had indeed noticed, yet.
The nude girl may indeed appear slightly confused at first, but then quickly get over it and claim she always had been that way in the pool. Or maybe even the coach may intervene and state that matter of factly, and the nude girl seemingly swallow that and go with it.
The main character may want to challenge that, he *knows* Nancy did wear a swimsuit last time, and possibly even no more than half an hour prior, but he realize, he fails to describe it, beyond a generic "same as usual" but it seems extremely implausible that it, the usual, was indeed nudity as claimed.
And the total indifference to Nancy's nudity may immediately be contrasted with the expected furor and giggling because of a near nipple slip or oven just exposed butt cheek of some other girl. Nudity is still very much not allowed, and teens still react to it sharply and loudly, except for when it's Nancy, seemingly.
That may not remain contained, but escalate. Maybe not right away, but the other day, Nancy may come to class nude. Again there's no obvious reaction. It may be possible to coax the acknowledgement that she's indeed nude from other people, but they all seem to accept it as normal, in contradiction to any posted rules or expectations. It's like our lead is only one feeling emphatically anxious about the Nancy's situation. But it's not like our lead makes lot of noise or it a problem either, they keep the inner struggle quiet with themselves, and be low key to not, well, make it a problem to themselves and most importantly to Nancy.
It isn't impossible so does anyone else. Someone else too may even admit it isn't "normal" but that they just run with it, because why not. Nancy may even herself admit in privacy that she's confused about it, but now incited to see how far she can go. Or not, she may keep claiming she's always been a full time nudist, despite the evidence... no, apparently, previous photos of her now show her nude indeed.
Then, the next day or week, Nancy is fully clothed, and seems to be totally clueless she had walked around naked, and so is everyone else. Our teller is ridiculed for mentioning it, or maybe... "Nancy? No man, you're so confused. It's Peggy who's our resident nudist, always had been." And indeed... he looks around for her, and Peggy is now nude.
No, it wasn't quite the intention to be Naked in School with mind control, but could be if you like.
But of course, it's very probable the main character may join in the nudity, in a number of ways, including, even much likely, suddenly noticing they are that way themselves. Or be noticed about it by someone, just the same way as in the beginning, but roles reversed. Or decide to try it voluntarily, and it works, or seems so. But was it volunteering, or they somehow were made strip?
Again, there's no need to even try to explain why or how it happens. The confusion and questioning of the main character's own sanity is the point. But yes, it all could be an elaborate prank with no magic involved. Or the main character may be the ignorant one, and it indeed is normal in this new school.