Any solution here is just an arms race. The better AI's get at generating text, the more impossible the job of identifying if an AI was responsible for writing a given text sample.
You could even just set up a GAN to make the AI better at not being detected as something written by an AI, I don't see a good general solution to this, but I also see it as a non-issue - if students have better tools they should be able to use them, just like a calculator on a test - that's allowed on tests because you still need to understand the concepts to put it to use