Does Stargirl have any conflicts with people? Who?

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.