I've been a high school teacher for 30 years, and I would agree that a teacher/professor/coach's job is to teach. No doubt. It is also incumbent on the students/players to learn, or to try to learn. Now, there are some students who just can't grasp what you're teaching, depending on how difficult/complex it is. I teach Spanish; I should be able to teach every kid, no matter what, their colors. Spanish has two past tenses, with countless forms. They are used differently from English. Kids learn/master this at varying levels, at different speeds, because it is extremely difficult, way harder than learning colors. Some get it slowly, some get it quickly, some never get it. If I were drafting a class to try to master learning this, I would pick the type of kids I think would be able to learn this. As teachers, we don't get to choose who we teach. The NFL picks the players they want to coach. It is incumbent on management first to try to find guys who they think can learn their systems. If the players can't, management has failed the coaches. If in a general sense, players struggle with a complicated system, then coaches have to decide moving forward if it is better to adjust, change, adapt what they are doing to the norm intelligence/capabilities of the player population. That is at least what I would do to start out. You can probably ratchet up the difficulty once you have your QB and some core guys on board, that you're not starting over with 8 fresh faces for a few years during a rebuild.
But to say that a teacher, professor, or coach isn't responsible for teaching is a bit off, I would say. I agree, the players need to do their best to learn; it's their job. But it's tough to learn things when they are not taught properly. Been down that road too, as I am sure many of us have.