This was my S20. He was simply not interested in getting good grades for the sake of making others happy. That, plus a nice does of ADHD, made him an underachiever.
That being said, he was abe to test well (like the OP’s classmate). He took the SAT once with almost no prep and scored a 1510, and had 5’s on most AP exams (despite getting B or even C grades in some AP classes due to failure to complete homework). He snagged a spot at a competitive top 50 national university, has a 3.4 in college (not as much daily homework helps), and will ultimately get a BS in chem. He wants to teach high school chemistry – and I actually think he will be great at it. He truly doesn’t enjoy the “rat race” and doesn’t measure success based on grades, US News rankings, or other traditional prestige factors. As a parent, all I want is for him to be happy and able to support himself.