My own experience, for what little it’s worth, is that sometimes you get students who undergo dramatic changes in their academic performance, for good or for ill, but those changes have nothing to do with intelligence. Someone’s performance drops because of illness or emotional upheaval. Or, conversely: Many years ago I had a student who took several classes from me and never got anything better than a C. At the beginning of his senior year he came to my office and asked me why. I reminded him that I had always made detailed comments on his paper; he said, yeah, he knew that, but he had never read the comments and always just threw the papers away. So I explained what his problem was. He nodded, thanked me, went away, and in the two classes he had from me that year he got the highest grades in the class.