What are some of the problems Junior has by deciding to go to school in Reardon？Why do you think he decides to change schools？

Junior decides to change schools in order to get the best education possible, which is very unlikely to happen at the school he attends on the reservation.
At his old school, in his geometry class, Junior realizes they are using antiquated textbooks; he is assigned the same book that his own mother had used when she was in school. The teacher, Mr. P., later becomes Junior's mentor and advises him to go "somewhere where people have hope." Junior concludes that this phrase refers to "where white people are," so he decides, with the support of his parents, to go to the best school in a white neighborhood: Reardon.
Junior faces a number of problems. His largest problem is his inability to connect with his new peers; Junior is an intelligent kid who is intolerant of less-intelligent people, and he struggles to connect with a predominantly white student body that has no bearing on his experience as a Native American on a reservation. Additionally, his best friend on the reservation, Rowdy, is lonely without him and accuses him of being a sell-out, leaving Junior without his closest friend.
Thus, Junior has to find a way to keep his old friend while also making new ones, as well as making sure to get the education he made these changes for. While he does not fully succeed, he does take huge strides towards claiming his own identity and directly confronting problems that his friends and family on the reservation face (which includes the tragic accident of his sister's death).

Willpinit is not an appropriate school for someone of Junior's educational needs: the facilities are poor, the textbooks are out of date, and the teachers don't seem to care. He thinks he would be far better off at Reardon, where they have science labs and computers, but Junior soon discovers that life at Reardon presents its own challenges. As well as experiencing racism from white students and staff, he's also regarded as something of a traitor by his own people on the reservation. They call Junior a "white lover," thinking that his preference for Reardon shows him to be rejecting his Native American identity.
Caught between two schools, Junior initially finds it hard to adapt at Reardon. To make matters worse, he's much poorer than the other kids and can't afford to go out with them in order to make new friends. At the same time, Junior's a smart, resourceful boy. Forced back on his own resources, he's gradually able to develop his natural talents in a way that would've been unthinkable back at Willpinit.