I am not a teacher, but I have made an interesting observation regarding classical mechanics education. It is surprisingly common that even students at academic level do not seem to fully understand Newton's third law (action and reaction) and confuse it with Newton's second law (equilibrium/motion), apparently due to the algebraic similarity when the net force is zero. An example of this is mistaking gravitational force and supporting force as an action-reaction pair. I believe a possible contributing factor to this confusion might be poorly chosen terminology in the English language. The force acting on a body is referred to as the "action" and the equal but opposite force acting on the other body is referred to as the "reaction". However, especially in engineering, the known forces on a body are referred to as "applied forces" or simply "loading" and the unknown forces on the same body (forces at the supports) are referred to as “reaction forces”. The semantics imply that the applied forces and the reaction forces are action-reaction pairs, which is of course untrue. In my native language (Finnish), the action-reaction pair is referred to as “force and counter-force”, and therefore the terminological confusion does not arise. What do you think, is the English terminology confusing?