Joining procedures and circuit flying are potentially the most hazardous bit of most PPL flying and I think it is incumbent on instructors to ensure that people are doing it safely and help them to do it better when they see poor procedures/knowledge.
Something I see frequently on dual checks, and from some surprisingly experienced pilots - frequent faults are:-
Turning against circuit direction within the ATZ
Descending to circuit height in the wrong place.
Carrying out a standard join when the brief tells you otherwise.