Stargirl is a self-named, homeschooled 15 year old girl, who decides to go to high school in order to make friends. She is different from her peers in so many ways. She dresses in costumes, carries around her pet rat, plays the ukulele, and is oblivious to social cues. At first, no one at school knows how to deal with her. Then, they embrace her and start to do the "weird" things she does, like have a flower in a vase on her desk, run outside and dance when it starts to rain, sing Happy Birthday in the school caf accompanied by the uke. Really what they were doing was trying to make her normal by changing what normal is. When this didn't succeed because Stargirl kept upping the "I'm different" ante, they shunned her. This was really difficult for her boyfriend Leo, who had been fairly popular before Stargirl entered the picture. On Leo's advice, Stargirl became Susan and started dressing and acting like all the other students. He was thrilled but no one else was. The shunning continued and Susan/Stargirl ultimately decides she's gotta be true to herself and she goes back to being Stargirl.
Stargirl had some very endearing qualities like making cards for people she didn't know to ackowledge successes, encourage perserverence, or share her condolences (all anonymously); leave loose change around for others to find; dance in the rain. She could also be annoying in her inablity to read social cues. She would sing to kids in the lunchroom (after the shunning) embarassing them. As a cheerleader for the high school basketball team, she rooted for both teams because everyone needs support. She put up a huge sign in the student lounge area proclaiming her love for Leo.
When Susan goes back to being Stargirl, Leo chooses his friends over her. He regrets this choice when he grows up. I think the story was too black and white. The message clearly is, "Individuality is good, conformity is bad (or at least a copout)" Life is a little more complicated. That's the whole difficulty of being a teen, you can see all choices out there and there are no easy answers. I think the simplistic story would have worked better aimed at a younger age group.