A student I taught multivariable calculus last year came back today. I always recommend that students retake it in college, to give themselves a breather course their freshman year. Most colleges don’t let kids pass out of the course anyway, strangely.
This student was in his freshman class, and said he kept on silently praising me, because he had an awful professor (and after hearing about him, I agree!) but my former student knew everything because he learned it all so well from our class. And all his classmates hated him because he kept on breaking the curve. Ha! Love it!