At basic flying trainng school I had a Flt. Sgt. pilot QFI (and he was brilliant), I was commissioned. At his suggestion first meeting of the day he would say "Good morning, Sir & I would reply good morning, Flight Sargent". The remainder of the day, on ground or in the air I called him "Sir" and he called me many things, including "mate", when I did fairly well !!
As long as the respect for knowledge and ability is recognised (both ways) I see little harm in two similar ranked people calling each other by any acceptable title (it is 2019). I guess that if a senior officer was involved (check ride, maybe) normal Service rules would apply. The students are there for one purpose, to learn the basics of air combat, and I know from several sources that it is working. I would love to give it a go again but the main objection would be the time factor, I would probably be about ninety on completion !!!!
Bill