Dude...
Run fast, run far.
Think about this: what if you're the best she's ever had? Orgasms produce oxytocin. Oxytocin makes women bond, (case in point:
http://www.hugthemonkey.com/2008/08/oxytocin-becomes-part-of-sex-ed.html) UNLESS they are aware, mature and sure (of themselves) enough to be able to separate love and sex. She's 18 - chances are there isn't enough life experience yet to be able to do that. So... if you're good, she'll want more. And more. There will be sobbing and tantrums and pouting and professions of undying love. "If you loved me, you'd MAKE time." If you're lucky, it'd stop there. If you're unlucky she'll try to connect with you more by texting, emails, status updates or - horror of all horrors - phone calls.
If you're bad, then she might get regretful and turn the tables and it could really bit your ass: "He seduced me. I feel so dirty. Boo hoo... he's older and I was taken advantage of...." then rats you out for sexual harassment when angry. Innocent until proven guilty? Not bloody likely...
YOU will see it as sex. SHE still has Disney-esque visions of princes swooping in on gallant steeds taking her into his big, strong man arms. (Can apply to any age, really). See above link. *
If you're going to have an affair with someone, rule #1 is always have one with someone with just as much or more to lose than you, otherwise, their sense of discretion isn't on par with yours, and they can only see the gain in being "outed," not the pain.
Go watch Fatal Attraction to scare yourself into flaccidness (normally not something I encourage... but under the circumstances

)
* and yes, I am aware that this is an over-generalization, but truth is, I'm old enough to have seen the above play out more than once, and more often than not.