Considering that many high school kids won’t want to put in any effort at all, how else do you convey the amount of detail and effort you expect for a given writing assignment? It’s an imperfect proxy but I can’t think of a better one.

Yeah. 1000 words is not a long essay that requires padding, and any competent teacher marks an essay with 1000 words achieved mainly by repetition and bad sentence construction much lower than one discussing the subject matter in a suitable level of detail, and probably lower than a better- written essay which gets marks deducted for only having 985 words.

Since "write an essay" can be anything from three paragraphs to a 50 page paper and the teacher probably doesn't think either is the appropriate response to the task, some sort of numerical guide is a good starting point, even if a fairly wide range is a better guide than just a minimum...

(plus actually there are real world work tasks involving composing text that fits within a certain word range, and since being concise and focused isn't AI text generation's strong suit, I'm not sure those work tasks will disappear...)

Yeah, this is seemingly the only effective proxy for "write with some amount of depth." If the word count gets BS'd then it will be obvious when reading the output.

When the teacher goes to grade it? If you turn in one sentence with or without a minimum your getting an F...