It is probably correct to say that one cannot damage an engine by excessive leaning so long as one is below say 65% power.
While one must go by the POH in all cases as the #1 reference, Lycoming have a general recommendation to not lean above 75% power.
So a simple rule like: never lean during climb and never cruise above [some power setting, an RPM figure on a fixed pitch prop plane] might do.
But I think not teaching leaning is the least of the shortcomings of the PPL syllabus
