Final answer:
The sentence that uses a simile is "The classroom was as silent as a library when the new teacher walked into the room," since it compares the silence of the classroom to that of a library using the word 'as'.
Step-by-step explanation:
The sentence that uses a simile is "The classroom was as silent as a library when the new teacher walked into the room." A simile is defined as an indirect comparison of unlike things using the word as or like. In this sentence, the silence of the classroom is being compared to the typical silence found in a library, using the word as, which fulfills the criteria of a simile.