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.

Yes, I would definitely say that Stargirl has conflicts with people.  I would even say that she is in conflict with a specific person as well.  

In general, Stargirl's main conflict is with most of the student body at her school.  Most of the students simply don't understand Stargirl, which causes them to shun Stargirl.  They are not necessarily always outright mean to Stargirl, but the students also don't go out of their way to be nice and include her in everything either.  It makes school a lonely place for Stargirl.  

Hillari Kimble is Stargirl's specific antagonist.  Hillari is outright mean to Stargirl.  Her mean streak begins with spreading nasty rumors about Stargirl and escalates from there.  At one point, Hillari physically slaps Stargirl in front of people.  

Shortly after, as the Serenaders gratefully played “Stardust,” Hillari Kimble walked up to Stargirl and said, “You ruin everything.” And she slapped her.

What is most interesting about your conflict question regarding the novel is that Stargirl herself essentially refuses to be in conflict with people.  People might have problems with her, but she doesn't have problems with them.  Stargirl is either oblivious to the antagonism, or she is so confident in herself that she simply chooses not to care.  I think it's a mixture of both.  Even when Hillari Kimble slaps Stargirl in the face, Stargirl flat out refuses to stoop to that level.  Instead, Stargirl kisses Hillari on the cheek.  

And in fact, when Stargirl finally moved, Hillari winced and shut her eyes. But it was lips that touched her, not the palm of a hand. Stargirl kissed her gently on the cheek.