Don't talk while the teacher is talking.
You musn't talk while the teacher is talking.
Scpecial clothes are not necessary.
You don't have to wear special clothes.
Put your phone on silent during the lesson.
You must put your phone on silent during the lesson.
It's not necessary to go to school early.
You don't have to go to school early.
Don't chew gum or bring food into class
You mustn't chew gum or bring food into class
Don't leave the class before the end.
You have to stay in the class untill the end.
Take off all jewellery before class.
You must/have to take off all jewellery before class.