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...)
My high school professors had a really good solution to this:
Minimum word lengths but you have to write the essay in class by hand. You have 2 periods.
Some of us still write a lot but having limited time and space (4 pages) really put a hard limit without saying so. In higher classes they started saying “I’m gonna stop reading after 3 pages so make sure you get to the point”
Demanding that students mind read is not a good strategy. Specifying expected length, checking for it is a good strategy. Teacher should also check for other things - whether paragraphs logically follow, grammar, sentence structure, you name it. But dont make them guess.