Why does Sam wear a star necklace?

I see no evidence in the text to explain the star Sam wears around her neck.

I think it’s probably related to the heavy amount of religious imagery and foreshadowing by Chbosky. This is quite inherent in the way that the football team is coined “The Devils” and Charlie wears a “Jesus Christ Superstar T-shirt” at one point in the film. Chbosky wanted to portray Emma as an angel per se, which is why her entrance is backed by a light on the bleachers, and why the fairy lights are behind her bed in her bedroom (also to foreshadow the stars) but this could also just be an artistic choice of fashion.