I would say that Junior's reasons for wanting to attend Reardan are valid. Here's why:
Junior's favorite subject in school is Geometry. As a freshman at Wellpinit High School, Junior is actually pretty excited about the first day of class until he realizes something: the Math book he is handed by Mr. P (his Math teacher) once belonged to his mother, Agnes. He realizes with a pang that his school is so poor that it has to reassign books that are at least thirty years old to new students. Subsequently, he becomes so angry at the state of schools on the reservation that he throws the book across the room, only to hit Mr. P square in the face.
Junior is suspended from school, but Mr. P assures him that it wasn't his idea to let that happen. In fact, he confesses to Junior that he has also wronged him and many other students on the reservation. When he was a young teacher, Mr. P had been instructed to dismantle or root out any inherent Indian characteristics his students possessed. He apologizes to Junior for this, telling him that he was young and stupid at the time, just like Junior is now. Mr. P tells Junior to leave the reservation for good if he wants to retain hope in his life and to hold on to his aspirations and dreams for his future. He doesn't want Junior to 'fade away' or to fail, like the rest of his ancestors or even his sister, who has given up hope of becoming a writer.
Because of this, Junior wants to attend Reardan High School, one of the best small-sized schools in the state. He tells his parents that the kids at Reardan are 'the smartest and most athletic kids anywhere,' and that's where he wants to be. He wants to be where he can hope for a better future and better opportunities in life; Junior knows that the only way he can achieve these goals is to venture out of his comfort zone. So, yes, Junior has pretty valid reasons for wanting to attend Reardan High School. He is greatly encouraged when his own father calls him a warrior, a fitting epithet for a young man charting his own path in life, irrespective of any obstacle in his way.