Yep - you just feed the AI a lot of essays by that age group, including the info that these are from 17 year olds, and it would do that.
Overall, people are not original, and teens even less so. Teens spend most of their time just trying to fit in and be like their friends, and that will come out in their writing as well.
Writing something that is both original and enjoyable to read is well beyond the powers of almost any 17 year old. If that were a requirement, there would possibly be enough students to fill the incoming class of one moderately sized Liberal Arts College, if that.
Thinking back on solutions, a possibility is that any essay has to be submitted with all drafts. While there are thousands and thousands of essays out there, there are maybe hundreds of examples of the first 10 drafts of an essay. Moreover, ChatGPT is probably not able to create 10 consecutive drafts which show changes, comments, and improvement.
Add to that an annually changing prompt, which will make it difficult for anybody to compile a database of tens of thousands of essays on that particular theme. Even if the prompt is leaked, there wont be a useful data set until after applications season is over.
There is also the possibility to use AI to identify essays written by AI. If there are specific characteristics of AI writing that can be identified by AI, based on a giant sampe of AI-generated essays, that would be the best tool.
Maybe humans cannot identify AI-written articles based on their experience in reading a few thousands essays. However, AI can go through millions of examples, and it is not limited by how humans read and process writing and language.
That assumes, of course that AI generated text does differ from human written text in ways that AI can detect.