Re: Instructors of low rank
Where I used to train in Japan it was literally an everyday occurance that a sandan uchideshi taught a class that was well attended by many 5th, 6th, and even a 7th dan. I thought it was a bit odd myself, so I asked. The answer was simply "it is his day to teach".
When I show up to Boston to see Gleason sensei and for some reason he is not there, I have had a wonderful time taking class from a person 3 ranks lower than me who took over class. I'm usually so happy to get to train and not have to worry about watching out for EVERYONE in the room like I have to when I'm teaching that I don't much care what is being taught and at what level of understanding. Who cares. I know what I need to work on, and I work on those things regardless of what techniques are being taught.
I would say that when I teach and people higher ranked than me are in class I don't normally correct them unless they want me to show them what I'm doing. I don't normally ask them up to demonstrate ukemi either - but if they want to do it, I might ask them up, and take their ukemi for the technique. It's all a bit silly really to worry about it.
I agree with the folks who suspect that the sensei for the OP is making a point about reliability.
The people who think that "rank" means ability to teach simply have not been around to many dojos where that is so clearly NOT the clase. There should be 2 separate ranking systems - 1 for martial ability, and 1 for teaching/coaching ability.
If the lower ranked student does try to make a technical point, I would hope that the senior student just take ukemi in the progressive resistance way and let the junior feel where they are straining - like you do for everyone else.