I remember having to bring a permission slip to be allowed to watch R-rated movies in class. This obviously isn’t a legal requirement, but it was a way to make sure the parents knew about it and were okay with it. And, well, if the parent wasn’t, then the kid got chucked out into study hall.
There are also places online where you can acquire the “family edits” of some movies (essentially the TV edits, but with things only removed for content and not for time). They tend to have words like Clean in the title, like CleanFlix or CleanFilms. But I don’t know how wide their selection is.