Sounds to me like your issue is with the high status placed on athletics in your community. This is a real hotbutton issue. Still, I think calling a teacher coach is appropriate. I dont think it diminishes the high esteem to which we should hold our teachers, and maybe it is, in their mind, a less formal address than Mr. or Mrs. so and so. You know, "Hey Coach, I'm not clear on that last point. Could you go over it again?" Instead of "Excuse me Mr. So and So, could you please repeat that last part?" Just maybe it makes them feel more approachable. 
