The Question and Answer section for The Absolutely True Diary of a Part-Time Indian is a great resource to ask questions, find answers, and discuss the novel.
Junior doesn't want to go because he doesn't want to take the chance he will be picked on or beat up.
From the text:
Gordy gave me this book by a Russian dude named Tolstoy, who wrote: "Happy families are all alike; every unhappy family is unhappy in its own way." Well, I hate to argue with a Russian genius, but Tolstoy didn't know Indians. And he...
Over the course of the novel, Junior loses both his sister and grandmother. He also loses his father's closest friend, Eugene, who is like an uncle to him.