gates

I think it is down to the instructor to make sure that everybody in the class understands the purpose of the grading system and maintains proper etiquette.

We have been told on occasions:

"Do it as your sempai shows you, even if you know it is wrong"
"Do not talk back to, or question your sempai"

I maintain this philosophy no matter which dojo I am in. I will always do the technique as the instructor showed first. If the sempai corrects me, even if it is clearly different than Sensei demonstrated then I follow their instruction. Sometimes Sensei will then come over and correct you, at which point you do not say "I knew that.." or some other smart arse comment", you just bow and say "Hai Sensei, Domo Arigato", these are lessons in patience, humility, and manors. Something that this 5th kyu sounds like he needs more than a good Iriminage.

The instructor needs to set/maintain the standard for the reigi (etiquette) in the Dojo.