How many teachers could justify the 5 paragraph essay if all the kids started questioning them on the arbitrariness of that number?
If you reward that kid that's really easy for you to take care off the others will follow, right?
Standardized tests measure your value because they measure the teachers value (in paychecks).
I have little respect for the school system. It's basically rigged to make the teachers lives as simple as possible and make the kids as miserable as possible.
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.