In literature, when we want to describe the tone, we're looking for what the speaker's attitude is toward the subject. In other words, how does the speaker of the poem feel toward what she's writing about? Here, the short answer is that the speaker's tone toward the poems and their power is one of reverence and amazement.
So how do we figure out what the tone is?
In any poem, and specifically in "How I Discovered Poetry" by Marilyn Nelson (published in 1997 in The Fields of Praise: New and Selected Poems), we can interpret the tone by looking at what the lines mean as well as how they express that meaning: that is, we should paraphrase the lines in our own words to understand what they're saying, and then we should pay attention to the individual words and phrases that Nelson selected, figuring out what kind of mood or attitude they convey. (Here, we're concerned specifically about the tone of the last two lines.)
When we do this, we'll discover that the tone of those last two lines is one of profound reverence—in other words, the tone is serious, contemplative, and impressed, because the speaker and her classmates feel amazed and deeply appreciative.
When I finished
my classmates stared at the floor. We walked silent
to the buses, awed by the power of words.
First, let's paraphrase those lines. After telling us that most of her classmates didn't care about poetry and were just eager for the school day to end, the speaker (the "I" in the poem) says that she finished reading the selected poem aloud to her classmates, who then looked down instead of at each other or up at her. Then, the speaker and her classmates leave the classroom to head home for the day, and instead of cheerfully chatting with each other, they say nothing, because they are "awed by the power of words," or impressed and amazed by the way in which words did amazing things in the poem they just listened to.
Next, let's look closely at the words and phrases Nelson selected in these last two lines. She chose short, simple words like "stared," "floor," "walked," "buses," and "awed," so she doesn't want to call our attention to any flashy words—she just wants the idea to shine. She just wants us to think about what the kids are doing and how they're reacting to the poem they just heard. With the words "stared" and "silent," Nelson adds a bit of subtle alliteration, letting the soft "s" sounds echo in our minds. Again, nothing is loud or flashy here in Nelson's word choice: the lines are almost as quiet as the children are. She dispenses with traditional grammar, writing casually that the students "walked silent," not that they "walked silently." The words sink deeply into these student's minds, just like the lines of Nelson's poem right here can sink into our minds, helping us envision the children and their astonished, impressed faces.