Jerry Spinelli's Stargirl is a novel about being an individual in a world of conformists. When Stargirl shows up at Mica Area High School (MAHS), no one can quite believe she is for real. MAHS is a place where being an individual is not a good thing. In fact, Leo says:
We all wore the same clothes, talked the same way, ate the same food, listened to the same music. Even our dorks and nerds had an MAHS stamp on them. If we happened to somehow distinguish ourselves, we quickly snapped back into place, like rubber bands.
Into this world of conformity comes Stargirl, who sings "Happy Birthday" to people in the cafeteria and wears outlandish outfits. Her most outspoken and openly antagonistic critic is Hillari Kimble--snobbish cheerleader and leader for the "elite" group at MAHS. She is the one who espouses the theories about Stargirl's presence at Mica High.
I'm not sure what, specifically, you mean by "conspiracy theory," but in chapter two of the novel Hillari speculates that Stargirl is part of a government plot. She continues her ridiculous and outrageous speculation by saying she was sent to shake up the student body and dispel the apathy which is so rampant at MAHS. In other words, she will give students something to actively rally around--or, in this case, against. Hillari is adamant that Stargirl is a fraud, and others believe what she espouses.