I'm teaching viola to a beginner high school student.
His progress with technique and reading are pretty good, but one thing he has trouble with is rhythm and tempo. He tends to want to work on expression and tone quality a lot -- which is understandable; a lot of beginner violinists tend to be more self-conscious about their tone (and these are probably the ones that end up excelling later on, given they don't become discouraged and quit). This is his first exposure to music performance, though, so tempo is more important than he thinks -- and he's struggling to grasp it or to appreciate its importance, as enthusiastic as he is about learning music in general.
I've tried a few things so far -- tapping to a metronome, counting subdivisions, playing a straight 16ths melody on top so that he'd know when he can guess intuitively from the chord change when his note comes in -- I've had moderate success with the last one, but he tends to rush ahead, especially during rests.
Are there any other tricks that I might try? I think it just hasn't "clicked" for him. Once it does, I'm sure he can pay more attention to it and either improve quickly with a metronome or at least over time with practice.