Obviously -- kind of -- lightly alternative reality fantasy setting for the main premise to even have a chance -- not unlike, say NiS universe, but the focus here would be on gradual influence and attitude shift and eventually acceptance not concentrate on some sudden, aggressive administrative violence although there surely could be some of that as part of the struggle between the old and new normal while the nudist area grows, both in time share and territory.
The change itself could be still rather fast, as in, within as little as few years going all the way from conservative prudishness to permitting and then accepting clothing optional lifestyle nearby, to seeing it as no big deal anymore, but perhaps not became universal either, as it is exactly the mixed, confused, time and space segregated amd simultaneously shared nature of the setting that makes it arousing, the change with each new individual convert not so much the end goal. And as for sex, there's not necessarily any or explicitly described, no much different from any other school story despite all the undoubtedly exciting and distracting nudity. Yeah, while it's a "stroke story" on it's own right the kink is mostly limited to growing acceptance of public nudity.
Well, that's too much meta already.
So, it could be...
... It used to be a not at all too large school of what used to be a country village, with probably had become a satellite and then had been recently overrun by sprawling exurbs of a larger city, but still independent and rather poor. Anyhow, since the school still basically in the countryside had lots of land under it and other near randomness of "optimizations" it had now quickly outgrown itself, many students now driving out here from the town not the other way, and most notably the sports facilities aren't adequate anymore, if there's even was any.
The sizable parcel nearby, between the school and the beach on the lakeside (or sea gulf, or real big river, anyway, area used as public beach, perhaps even partially developed beach (most possibley we're in a year-round beach weather land, but not necessarily)) had been bought for building a resort with a water park and what not. And part of the deal allowing it there was that they will also build facilities for the school, and as compromise, somewhat shared facilities, the Olympic size pool including.
The project was already nearing completion when the company behind it folded (because of related difficulties elsewhere) and it was bought over by another operator specializing in, yes, nudist resorts. And they really liked the purchase even if they would need "specific" access to at least part of the beach and while town hall was outraged by the idea at first, a compromise was eventually reached. While some clothes optional areas would be set aside with strict separation and kept low key, the company agree to do even more for the town, including the school.
But it takes up big, the new destination turns out to be popular and their slice of beach overflows, and is officially expanded, and then doubled again. Soon it's a textile end on a nude beach not the other way, especially after the resort buys out nearby ranch or two and expand into the new land quickly, including a deal for access into wilderness park behind. And then the new housing development jumps in and promotes the resort nextdoor as a perk and ditch plans of walling it off, at least in the resort side, basically joining on in, and turns out it sells and they are looking for more land too. And meanwhile on the townside beachfront, small shops start realizing that requiring all clients to wear shirts isn't reasonable anymore. And the town hall has to admit that while it may not be really separated from the overall suburban growth in the area, the resort is helping the bottom line even if they may not agree on the specifics.
By this point the school is surrounded by clothes optional land on at least three sides and is basically rebuild by the resort operator, with town hall indebted. And the back of the oversized school land, always much too big and now wholly unnecessarily for any future sports fields as students have direct access to very good facilities on the resort (on a strictly separated schedule, mean you, at least officially) is now seen as undeveloped and underutilized land in the middle of the resort and a deal to develop and integrate it, even probably without owning it is being sought, and eventually reached.
And a big point is that one the fence comes down, the new one planned close to the school never appear. And really what's the point, the separation had been growing fuzzier for a while. While school may not encourage overstaying in the facilities on the resort side after official school hours, by the resort they are welcome. And no, nudity isn't strictly enforced, just customary.
And then there's new students who have moved in the area who are indeed from dedicated nudist families. And while they don't openly resist rules still requiring some coverage in the classroom, it start to seem more and more pointless. The natives are divided as some are joining in and some in opposition and the city kids coming out from town curious about opportunities.
~
Something like that, and sure, it's almost all almost entirely the background, not necessarily even known or understood by the protagonists. It's just too easy to build a world, actually placing a story there is something else. And should I remind I don't even speak English?