Could you take the matter-of-fact but still hard-nosed approach and take her to get checked out for an STD panel, given implanon, given the HPV vaccines and just say that you want to make sure at least if she does have/is having sex she won't be passing anything on to her partners - casually mention how embarrassing it is to have to tell your partner to get checked out for something you may have given them, and how teenagers often take great delight in spreading gossip about who has what STI and etc, and basically scare the living daylights out of her so she'll at least be sure to use protection if she does decide to have sex?
Maybe that's too devious and underhanded though - I'm not a parent of a teen so not really qualified to speak. Though that's pretty much the tactic used with me and it worked brilliantly.