[Xylakant]

You have 20-30 kids with varying backgrounds, skill levels and learning habits. Some require challenges to figure out things on their own, others explicitly explanations. Some work well in a group, others need individual attention. Some go through a rough patch at home or with friends and are distracted. Some days are hot and you make no progress.

A teacher needs to respond to the dynamics in a large group of non adults, every day, every minute. You can’t plan that out in advance. Sure, experience helps to make the planning easier and to respond to situations you’ve seen before, but still, every day is different, and responding to the challenges in the last lesson requires a plan.