It is very possible to stall and be climbing/descending/high speed/low speed. And in any attitude.
If you are teaching high RoD as a symptom of a stall you are misleading your students. Now a spiral dive does produce a high RoD and if you are teaching high RoD = stalling then guess what the student will do?
Whats wrong with teaching them to recognise the onset of buffeting instead.
Btw what produces a higher RoD a sprial dive or a spin?