I think that Reardon represents a significant moment for Junior. He sees his entrance into the school as a crossroads of sorts. Given where he is in his development and his age chronologically, if he does not make significant attempts to define himself as someone who can envision life off of "the rez" at this moment, he might never get the chance to again. Junior understands that there is an inward pull to the reservation life. It's like an undercurrent of an ocean. If one does not actively resist it, one will get carried off by it. Junior has already made it clear that he does not want to become victimized by life on the reservation, so he is willing to deal with all that comes after him with life in Reardon.
Transportation and merely getting to the school is difficult enough. The classes are challenging, as the curriculum and teaching is far more advanced and driven than what Junior encountered on the reservation. Additionally, he is socially "different." In a setting where there is only White people, he stands out. In his life on the reservation, he stood out because he refused to be clumped as a part of it. In Reardon, he stands out because he is not White and is fundamentally seen as different. He recognizes that he is in a different world socially and those expectations help to form his character as one who straddles both worlds, where his "Spirit" is one that chases the best out of both.