I'm writing a story and I'm trying to find words/phrases to describe the more innocent girl. These girls are all sophomores in high school, and this girl is kinda like the baby of the group that they are all protective of. She isn't a goody goody, or at least she doesn't try to be, but she's just more careful and shy. Hopefully this make sense? Any suggestions?
"Nice" would probably be one of your more useful terms for this character. In the hyper-competitive world of teenage girls, kindness and compassion are lost in pursuit of clique status and coolness. A young character who innocently facilitates her friends' happiness instead of her own, will immediately be seen as diminutive to her more stylish, more selfish peers.
It is best to describe such a character through her behaviors and reactions, rather than through the voices and opinions of her friends, but when verbal description is required, "Nice" is a powerful word choice. It is simultaneously complimentary and derogatory. It is exactly the type of double-speak which teenage girls love; easily saturated with scorn and ridicule while remaining harmless and even kindly upon review.
It is also useful to leave the description unspoken. "She is so..." is a very teenager thing to say, either caused by limited vocabulary or an unwillingness to commit to an opinion. Adding incomplete dialog like this and having other clique members affirm it without it every being directly stated, allows you as author to show consensus within the group. It also allows you to more clearly describe what is meant by the obscure comment within the private thoughts of the point of view character. Those thoughts, which are free from the limited vocabulary and voice of youth, can more clearly describe both the innocence of the target character and the protective affection which everyone holds for her.