In The Absolutely True Diary of a Part Time Indian, what pushes Junior to make the decisions to go to Reardan?

Anger and hope are what push Junior out of his life on "the rez" and into the world of Reardan.

When Junior starts high school, he is filled with excitement.  He believes that this is the start of a new life filled with hope for the future.  However, these hopes are dashed when he is given an "old, old, old...geometry book"  from which he is to learn.  He recognizes that the book is his mother's...

Anger and hope are what push Junior out of his life on "the rez" and into the world of Reardan.

When Junior starts high school, he is filled with excitement.  He believes that this is the start of a new life filled with hope for the future.  However, these hopes are dashed when he is given an "old, old, old...geometry book"  from which he is to learn.  He recognizes that the book is his mother's and is over thirty years old.  His excitement turns to anger and rage, filling his heart "with the force of a nuclear bomb," while his "hopes and dreams (float) up in a mushroom cloud."  At this instant, Junior is filled with intense anger because he recognizes the limitations of his education on the reservation.  He knows that if he stays on the reservation, he will never become the person he wishes to be.

Anger ends up giving way to hope.  After Junior assaults Mr. P with the geometry book, he shares a profound conversation with the geometry teacher who tells him that Junior is similar in talent to his sister and that he is "a bright and shining star...the smartest kid in the school."  Mr. P confirms what Junior already knows which is that life on the reservation is one where Native Americans are taught to "give up."  It becomes clear that if Junior wants to have a meaningful life he must "go somewhere where other people have hope."  

This realization inspires him to go to Reardan.  He understands the risk involved as he will be one of the few Native Americans there.  However, to do what he wants to with his life, Junior knows that he must pursue his education at the new school.  He is filled with hope when his father drops him off for his first day, calling him "brave" and "a warrior."  His brave resolve underscores how Junior's anger has been channeled back into hope for the future.