My girlfriend is apprehensive about our relationship, help!?

I'm 17, 18 in a month and my girlfriend is 15. I'm staying in town for collage and I think we can make the relationship work. She thinks otherwise. She thinks things would be too weird with me in collage and her in high school. Her parents are also apprehensive because of the age difference, but then again, they don't know me that well, and I'm sure I can show them that this relationship can work, and work well. I'm not worried about the legality of our age difference, because in the state I live in, if you are over 18 you can date up to five years younger than you, at least, that's what I have heard. Despite this law, I doubt I'll get pinned with statutory , because she is not ready for , and I definitely won't push her to it. The real problem is that SHE thinks the relationship won't work with me in collage and her in high school. I have to prove her wrong, but I don't know how......