"Knock, knock." Who's there? "We don't really know." We don't really know… who? "Right—we don't really know who the speaker of this poem is." In fact, we're not sure if it's a he or a she, if it's someone in the class, or if it's just an omniscient being that is aware of what happened in class one day.
Imagine you're driving a taxi, someone hops in the car, and starts telling you this story. It's like that. We're overhearing some retelling a story and we don't really get any info about the speaker. Notice the opening:
After the teacher asked if anyone had
a sacred place
and the students fidgeted and shrank (1-3)
The speaker doesn't use any personal pronouns. He doesn't say "we fidgeted and shrank." In fact, there's no "I", there's no "we", and there's no "us." Instead, the speaker is removed from the action of the poem, and is merely an observer.
It could be that the speaker is the teacher and is just referencing one of his own experiences teaching but using the third person to do it. But one thing we do know is that the speaker is pretty smart. He's somewhat removed from the action of the poem but is able to take us along for the ride, so to speak. He's able to recall with careful detail what seems significant: the students fidgeting, the car, the dashboard, the music, the key, putting the key in the lock, etc., etc.
What effect do you think this creates? Rather than the "You weren't there, you wouldn't understand," we get a speaker who has a deep understanding of what's happening. In fact, it's almost like, though the speaker's telling, the poem becomes a parable that we can all learn from. The speaker understands the serious student's revelation, and he (we're just guessing it's a he) wants to make sure that we, as readers, understand his insights, too.