I've a rule to let my kids question anything once - they must provide a rationale if they wish to reverse my decision. However, it does sometimes come down to a simple exertion of authority (ie because I say so trumps the unsupported objections). I find now that J at 6 can usually answer my side and rationalise why an a priori arbitrary demand is being made.

Surely any teacher could readily answer a challenge like 'why not 3 paragraphs?' and this is a valid and worthy question for a student.

Unfortunately at some point the student will have to accept irrational or limited reasoning and learn to live with that, eg why does the law require me to learn that.