Age-gap in Off Topic
I wouldn't recommend it. I mean, if you really must... but remember she's not fully mature yet. So don't get too involved, and be really really nice.
Also keep in mind that if anyone at your school finds out you'll be labelled a pedophile. =P (I've seen it happen, and that was only because a Year 9 guy said that he thought a Year 7 girl was hot. Did he regret saying that...)