What does the girlfriend's family structure and parental income-level have to do with whether she knows much about poly love?
The reason she isn't ready for it is because SHE'S IN HIGH SCHOOL.
Respect that she has certain boundaries and certain things she feels uncomfortable with. Don't push her to cuddle together or to talk more with you.
I totally get what you mean about feeling fulfilled and free for the first time because of being in a poly relationship. But I would not have gotten it when I was 16 and a virgin.
For that matter, your fiance seems kind of young to be engaged. What's the rush? You are all young enough to be able to figure things out slowly.
Single, straight, female, solo, non-monogamous.