The teacher will have to know the students better I suppose. If suspected of cheating a student should be given the chance to provide a writing sample and demonstrate knowledge in a face to face interview. It seems like the assignment here was just a summarization of some body of knowledge being taught; not something requiring original insight. It is the original ideas and analysis that, at this point, seems out of the reach of ai. And I guess more assessments of students should be testing the ability to produce such original insight, rather than what amounts to just a test or comprehension.