School as a symbol
In a way, school literally is a symbol of the population, but in this story that is especially true, because these young characters find themselves encountering the unknown in people at their school who treat them in confusing ways. In a way, school becomes a symbol for how one views their own self, because school symbolizes a person becoming, and so when Sone figures out that her school doesn't like her, it's hard not to feel crisis, because she doesn't understand what about her is socially unacceptable.
Racism and the motif of mistreatment
The memoir depicts Sone's experience of racial prejudice in her community. Although overt racism was somewhat rare, it was normal to see only angry faces from the other kids at school. The racism is depicted in the motif of how the school mistreats Sone, because the school is harsh toward her, and she doesn't know why, but eventually the answer emerges. Where she lives, Japanese people are subject to racism.
Asthma as a symbol
Although the fact could be technically true, Sone's mention of asthma is still symbolic. In America, the "air is bad," so to speak, so she struggles to live there. That's the heart of the symbol; that her experience of life in America is riddled with concern and fatigue, like she can't quite catch her breath. Asthma represents the unseen obstacles that she faces.
The death of the brother
The true stakes of negative emotion seem ambient to Sone until one day, her brother dies tragically. To her, the death represents death to be the stakes of human life. Her experience of racism is concerning because of death, because she feels chronically unsafe. Although death seemed too extreme to mention in her real life, the death of her brother makes the comparison mandatory for her. She cannot ignore death.
Pearl Harbor and internment
The idea of these symbols is paranoia, at least in these memoirs, because the racism from her childhood, alongside the death of her brother, made Sone more likely to suspect something horrible would happen to her, so then when Pearl Harbor happens and her family is rounded up by the government and detained in a concentration camp (yes, that happened), she feels her fears were justified the whole time.
