What is one way you have seen a teacher assert their authority over a class?

I've seen a teacher assert their authority over the class by threatening to send students down to the office or call parents for bad behavior. In my younger years back when in elementary they threatened to not take the class to the playground during breaks. (Sorry if it wasn't the exact answer you were looking for!)

By explaining what the teacher can and will not tolerate in the classrom. Raing of the voice.