Say "jailbait". That usually works. Just tell her that if she did that when she got older it'd be more okay but in highschool shed be jailbait and people would think she's sleepin with him to keep him and that she must be good and easy.... Sorry to be blunt but that's the many things I've heard about people. especially about the athletes and their girlfriends.
Still. I'm not too worried about it. Some other freshman dated a senior and people actually approved of the relationship.