Some teachers have been trained on how to enter a room so that they will create a presence. That means that (hopefully) they will be noticed. They are taught how to play the game.

A teacher with the right amount of presence will seat the children reasonably quickly. However, the class cannot begin yet, because the students are still involved in intensive chattering. The weary teacher has to quiet them down.

The children don't necessarily have anything important to say. However, it is very important for them to continue talking as loudly as possible at this point so that they can add to the delay in beginning their class. They avoid looking in the teacher’s direction during this time. After all, they don't have to be quiet if they do not hear the poor teacher begging them to quiet down.