I understood this beginning in my fifth-grade math class, when I started bringing in books to hold in my lap and read while the rest of the class did busy work. The teacher had created 'packets' of repetitive worksheets and students were supposed to collect them and fill them out, then she would sit at her desk the entire period and grade them. You got a star by your name on a wall chart for each packet you completed. I got the first packet to sit on my desk as a prop, but never wrote on it. That was the whole class, even as a child I recognized this had to be the dullest and laziest teacher ever. I tuned out and read some great books instead. The math wasn't that hard and I did okay on my tests.

The fun part was several months later when the students began to notice that while everyone had dozens of stars by their names, one student had none at all. One of the other students raised their hand and asked her about it in front of everyone else (I was amazed the teacher had never noticed). She was upset and berated me about it in front of the class, then sent me to the Principal's office. I knew instinctively that she was totally fucked for being so clueless, and I happily explained exactly what I did to the Principal and later to my parents. I wasn't in trouble and I knew it. About a week later a new teacher was brought in and she was great, she interacted with the class and I started participating because class was once again more interesting. I don't know how a teacher that bad ever got hired, but at least the school did the right thing later.