The lazy student. The student who, for whatever reason, does not practice; the student who gives nothing back. The longer it goes on the less likely they are to ever start. In a class situation they are tolerable, but privately (and it is strange how it's always the lazy students that demand private lessons – as though that privileged exposure compensates somehow for lack of practice) they are, believe me, far more trouble than they are worth (than the money they pay you).