Why does the community get mad when Junior makes the decision to change schools?

When Junior decides to attend the affluent white school in Reardan (twenty-two miles away from his reservation), his parents are happy for him but also anxious about his future. First, they are concerned that no one on the reservation will understand their son's decision. Second, they are worried that Junior will be stigmatized by his determination to go against the prevailing public opinion on their reservation. Third, Junior will be the first person to leave the reservation for anything.
For an indication of why the community reacts negatively to Junior's decision, let's take a look at Rowdy's reaction when Junior talks to his best friend about attending Reardan together. Remember that Rowdy and Junior have been best friends for fourteen years; they are almost like brothers and have been involved in the football, baseball, and basketball teams at Wellpinit for many of their years in school together. When Junior breaks the news to Rowdy, Rowdy is initially incredulous. However, when he realizes that Junior is serious, he becomes violently angry. His anguish is expressed in a terrible and prolonged scream. Junior states that his best friend's scream is the worst thing he's ever heard.
To make matters worse, Rowdy accuses Junior of thinking that he's better than everybody else on the reservation. Junior denies this, but Rowdy refuses to be placated by his friend's assurances. Grieved beyond measure at what he thinks is Junior's betrayal, Rowdy punches Junior in the face before walking off. Rowdy's reaction highlights his fear of the unknown and his anguish at losing his best friend. Perhaps there is the fear too that Junior will forget his Indian roots and the great relationship the two friends have shared for many years. More than anything, Rowdy's reaction highlights the prevailing Indian belief that only white people are destined for success in life.
So, any Indian who tries to change his destiny is immediately perceived to be arrogant and presumptuous. Perhaps, on a deeper level, Junior's community is merely afraid that there isn't any real possibility for someone from their reservation to be successful in life.
Because of their fears, they feel threatened by Junior's determination. Additionally, Junior's community may also feel betrayed by the fact that he has chosen to attend a white school in lieu of a reservation school. For his part, Junior knows that he must overcome his community's disapproval in order to be successful; because of his goals, he's willing to travel to where the opportunities are, even if this means attending a white school and upsetting his community.