Any number of scenes could provide a variety of answers to your question, but I'll offer one.
Junior's first day at Reardon is certainly an important event and transformation in the novel. In this scene, all of the apprehensions and anxieties of crossing the divide from one culture to another come to fruition. Junior is initially ignored by the white students but the taunting and threatening remarks soon follow. Of course, these tensions continue to boil within Junior an eventually he is driven to violence is retaliation.
So much of this novel is about living caught between two radically different worlds. This scene when those tensions become a physical reality is certainly one of the novel's most important.