Be sure to repeat this quieting procedure each time you deem it necessary. This not only allows you to get away with not knowing the material, but encourages the students towards finding the answer on their own. The students expect a Substitute Teacher to be professional. This means dressing, talking, and acting professionally. This rules out dressing in blue jeans and ripped tee-shirt, and cursing.
Different schools have different rules.
It is your duty to know the different rules of each school. Being a Substitute Teacher and being flexible are one of the same. You must be willing to teach a Boy's P.E. Class, even though the Gatekeeper called you in to substitute for an Advanced Trigonometry Class. If the Sub is not flexible, the school may have a hard time effectively utilizing the talents of this Substitute Teacher. Let this student sleep.