For most of Jerry Spinelli's Stargirl, Susan Caraway (aka Stargirl) does not care in the least what anyone thinks of her. Though her mother is a costume designer, Stargirl wears what most consider to be odd or quirky clothing because that is what she wants to wear. She sings happy birthday to people in the cafeteria during lunch, puts a flower in a vase on her desk each day, and generally acts in ways her classmates do not understand or appreciate.
We wanted to define her, to wrap her up as we did each other, but we could not seem to get past "weird" and "strange" and "goofy." Her ways knocked us off balance.
Because they do not understand--or want to-- Stargirl is teased and taunted, but she does not mind and continues to care for others more than for herself. Even when she becomes a cheerleader she is unable to fit in with her peers, She cheers for the other team and cannot bear to hear her classmates boo their opponents.
The one time she does try to be like everyone else is a miserable failure. Stargirl is just not like other people, and because of that she will never fit into their often artificial and rather unkind (okay, cruel) world. Her transformation does not last for long partly because it does not gain her any friends but mostly because she is too free-spirited and individualistic to conform to other people's standards for very long.
The truth is that Stargirl is not the one who changes in this novel; it is the others who change as they interact with her. As you write your essay, you will probably struggle if you try to write about Stargirl's trying to fit in with her peers; instead you might want to talk about what it means not to succumb to peer pressure if you want to write about Stargirl.
On the other hand, Leo is a perfect example of doing awful things as he succumbs to peer pressure in order to fit in. The worst thing he does, of course, is tell Stargirl she has to change, something which, at least indirectly, causes her to disappear. Leo is the one who has lessons to learn about himself and his relationship to his peers; Stargirl already knows how to be her own person in all circumstances.