Answer:
The given sentence is in the active voice. To change it into the passive voice, the object of the active sentence needs to become the subject of the passive sentence, and the verb needs to be changed to its corresponding form of the passive voice. Therefore, the passive voice of the given sentence is:
"The students' follies into passive voice were being laughed at by the teacher."
In this sentence, "the students' follies into passive voice" is the subject of the sentence, and "were being laughed at" is the passive verb phrase. The preposition "by" is used to introduce the agent (the teacher) who performs the action.