The poem follows one boy through three stages of school: elementary, junior high, and high school. In each, the pattern is the same. He is shy, has no friends, and is mocked by the people around him. In junior high he copes by blocking out the cruelty with music in his headphones, but by high school the bullying only gets worse ("more and more").
The ending is the hardest part. When the speaker says "Where is he you might say," the answer is blunt: "he's in grave." The line that he is "happy as can be" is bitterly ironic. It suggests the boy only found peace in death, and the phrase "Thanks to all the people who called him names" turns that peace into an accusation against the bullies.
The mood is sad, quiet, and angry. The plain language and repetition of "There was a little boy" make it feel like a warning story, and returning to that line at the end leaves the reader with an unsettling sense of loss.