2. digitize & feed to learning model, which detects that YOU are cheating.
upside: this also helps detect students who are getting help (e.g. parents)
downside: arms race as students feed their cheat-essays (memorize their essays?) into AI-detection models that are similarly trained.
That said, there's no need to consider changes in years when stylistic choices can change from one day to another depending on one's mood, recent thoughts, relationship with the teacher, etc.
That's why I've always been a little confused about how some (philologists?) treat certain ancient texts as not being written by some authors due to the text's style, as if ancient people could not significantly deviate from their usual style.
Initially I thought you meant having the student write an essay about slurs, as the AI will refuse to output anything like that. Then I realized you meant "N" as in "Number of words".
Still, that first idea might actually work; make the students write about hotwiring cars or something that's controversial enough for the AI to ban it but not controversial enough that anybody will actually care.
Downside: it also likely detects, without differentiation, students whose writing style undergoes a major jump because of learning, which is, you know, the actual thing you are trying to promote.
Why once? Most students need writing skills more than half the high-school curriculum.