Depending on the reader, a few themes might emerge. That's the nature of poetry. For me, I think the theme is a coming of age theme. A story/poem about a loss of innocence.
The main character is a 10 year old boy. The reader is introduced to that boy when he is finding out that his father is dead. Upon first reading, it looks like the poem is about grief, but after a closer examination, the reader can see that the boy isn't that sad about his father's death. The boy is away at boarding school and young, so he likely isn't that close to his father. So grief is ruled out. Especially when the boy himself tells his audience that he plans to use the grief to shield himself from bullies. The grief gets him attention, which he then feels shameful about. He used his father's death to get attention. The loss of innocence theme is because he, as a ten year old boy, now realizes that he can use things like grief, death, and shame to manipulate people.
The pride like goldfish line is better understood with the previous goldfish line: "Some goldfish in a bowl quietly sculled around their shining prison on its shelf. They were indifferent." More or less this is saying that even though the goldfish are in a prison for all to see, they don't care. I think the boy feels pride like a goldfish, because he, like the goldfish, doesn't care about what others think of his grief or shame so long as he can use it.