Agreed re: range of how people can be effective teachers. Sometimes, in an interactive setting whereby students can offer feedback or participate in conversation, it's possible to nudge a teacher into better performance, but fitness classes don't strike me as one of those settings unless there's a comment box or one feels comfortable saying something politely to the teacher's face after class.

Yeah, a teacher's lack of ability is rather harder for a student to fix, except by going elsewhere (though one runs out of classes to try eventually). :/