The original question had to be edited. I would suggest that the main conflict in the novel is how to navigate social assimilation in the high school setting. This becomes critically important for Stargirl and Leo. Both of them are impacted by this conflict in different ways. Leo's reaction to it is something that he does not fully understand until later on in his life. Stargirl realizes that even if she tries to subdue her own identity, she will never be able to fully become what others want. Her own need to be herself is stronger than any other social force of conformity.
The conflict of fitting in at the cost of losing one's own essence is resolved by Stargirl's repudiation of such a condition. The ending of the novel is one in which Stargirl does not capitulate to what others want or demand. She recognizes who she is and what she believes as more important than anything else. Leo understands this too late, beyond high school. He recognizes that social conformity is contingent and transitory. The ability to be who one is represents something universal. It is in this realization where the main conflict is resolved in the characterizations of Leo and Stargirl.