Tim, so much depends upon your daughter. Is she open, friendly and gregarious or is she shy and reserved and doesn't make friends that easily. If she is shy then i would definatley suggest that she bring a friend if her friend's parents can afford it. If she is bubbly and outgoing she will have a great time doing all the teen activites during the day and may just want to relax and hang out with Dad for dinner.
My top two picks for cruise lines for teens would be Princess(Grand Class Ships) and Royal Caribbean (Voyager Class Ships) book during school break and there will be plenty of teens aboard.