The correct answer is the first one: "The entire senior class and several chaperones attended the dance".
In passive voice, the direct object of the sentence in the active voice becomes the subject and the subject of the active voice becomes the object or it's dropped.
Considering this, we can conclude that the correct answer is the first one since the object of the original passive voice sentence ("by the entire senior class, as well as several chaperones") has become the subject of the new sentence, and the subject of the passive voice sentence ("The dance was attended") has become the object.
