What is an important topic featured in Spinelli's Stargirl?

One of the most important topics featured in Stargirl is whether people should conform to social expectation. 
Stargirl is a nonconformist at Mica Area High School.  Leo and the other students "did not know what to make of her."  Stargirl does not conform to any of their expectations.  Her uniqueness is not immediately understood. Her displays defy expectation.  For example, Stargirl incurs public scorn when she comforts an injured athlete from an opposing basketball team.  Her dress and quirky behaviors such as signing "Happy Birthday" at lunch are not enthusiastically received.  However, Stargirl is going to be herself.  She represents how people can be different if they have the courage to embrace this part of their identity.  There is a part of Leo that wants to do this. However, his actions show that he is not able to live a life based on nonconformity.  His lack of faith in himself leads him to reject Stargirl.
Leo's reflection about Stargirl shows that he regrets his embrace of conformity.  As the narrator, Leo does not reflect fondly on the people who  embodied the social landscape of Mica Area High School.  Rather, he reflects on Stargirl, the person who challenged social reality. Spinelli endorses the nonconformity topic on two levels. Stargirl's personality repudiates conformist notions of the good.  At the same time, Leo's fond reflection about Stargirl underscores the importance of challenging social conformity.