I just graduated from high school in May, and I do have to admit that there have been a couple of girls that I have been interested in. However, I knew myself enough to know that I wasn't ready to start "dating".
My sister (who is a year younger) started dating one of my friends back in August of last year, a relationship that lasted until this last April, and when it ended, the waterworks began, this coming about a month after they had just faked a breakup on facebook. When they faked the breakup, I knew the whole time it was fake, so I posted some seemingly insensitive comments, not the least of which was a link to a video of the Hallelujah Chorus. Eventually this lead to a discussion amongst myself and one of my sister's friends about high school relationships (which as a general rule I am opposed to). The sad realization I came to is, very few teenagers use dating for its original purpose: to find a spouse. Rather, they view dating as a means of having fun and fulfillment, and all to easily allow their relationships to become sensual and hormonally controlled. When this happens, it is nearly impossible to reason with them, as they are now making their decisions based on feelings caused by those nasty things called teenage hormones.
To answer your questions, are you the HS student that's smart? no, but you are in a select group that is. I am lucky enough to have several friends that are the same, but I also have friends (most of which I haven't seen in several years) who aren't, specifically one 20 days younger than me who got engaged late last year.
Finally, what can you tell her? In reality, they probably isn't anything you can tell her to change her mind, the only thing that you can do is pray for her. Pray that God will show her how foolish she really is for thinking that she is ready to commit to marriage at such a relatively young age.