> The assessment should be done elsewhere, by someone who is not responsible for educating anyone.
There's room for both educator-offered assessments and third-party assessments. As I said before, educators need assessments anyways. We have a fair amount of effort that graded assessments from educators predict future performance better than these third-party assessments, too (of course, the best predictor is both together-- e.g. SAT plus grades).
> Under this structure, it would be possible for cheaters to mess up the teacher's assessment of their proficiency, and by extension the overall proficiency of a group of students. But it is strange to assume that they would do this, because -- I asked this above -- what benefit would they derive from doing this? If this action had any effect, that effect would be to move the curriculum onward from material they don't understand to material they understand even less. That is a harm to the cheater. The problem just doesn't arise.
No one looks at middle school students' grades except the students and their parents-- well, and for eligibility for participation in athletics. Still, there's rampant efforts to cheat, with the negative outcomes I described above.
You're attributing a whole lot of foresight to a short term selfish decision. Yes, cheating may eventually bite you, putting you into a class you're unprepared for. But it provides a lot of short term apparent benefit, so...