I dunno, I think it would be weird. You might not notice it at the time, but you grow up so much during your teenage years. I've just finished my A Levels and I'm not the same person I was in year 9/10/11, or even year 12 really by a long way. Even people in the year below say things that make me think they're slightly immature (and I'm sure that's what the year 13s thought about us last year, etc), so I couldn't imagine going out with someone much younger at all. When you get to sixth form your priorities are completely different to that of a year 9 or 10, or even 11 really. Unless she is exceptionally mature for her age, I don't think that you'd ever really find the understanding of one another necessary for a proper relationship.
Also, when I turned 18 my social life changed quite a lot. I can now just go out to the pub or a bar with my friends for a few drinks and to catch up, and ditto with my ex-boyfriend. She won't be able to do any of that, at the moment she technically can't even go and see anything rated more highly than a 12A at the cinema for god's sake, and I think all of that could get really frustrating.
And like others have said, even if you did manage to overcome all those obstacles and your relationship progressed to a stage where you were ready to have sex, etc, then you would be breaking the law. And also, I think at 17/18 you're going to reach that stage a lot quicker than she would, it's just natural, and that too could get frustrating for you. I know you've said you don't want that from a relationship, but sex, etc, is a big part of a serious relationship, starting from people aged about your age, and I do think it would become an issue at some point, even if you're not thinking about that now.