BP, I have to disagree with the thought that this is an unnatural attraction. Both are in high school, likely have similar tastes in music, thoughts on life, similar life experiences and references, and a common basis for thought and understanding. If the 17 year old were in college I might agree. But, I am assuming they both attend high school and likely at the same school. Anyone who spends time around these kids should know that they do share thoughts, ideas, experiences, music, and concepts - the basic building blocks for any relationship.
Yes, the younger girls might be unduly smitten with the attraction of a senior or older guy, but this is also natural. If you think about the socializing aspects all of this works as natural. Where we differ from the alley cat is that - in theory - our brains should warn us away from behavior that is both immoral and illegal ... or, at least illegal. But, if teenagers have two things in spades it is hormones and an overwhelming lack of common sense. And that is where parents need to step in for the sake of the kids. If the kids are incapable of exercising the mental acuity necessary to overcome biology, then we need to act to restrict or limit the opportunity for such actions ... or, provide the increased possibility of consequences for such activity (i.e. deterrence theory ... a problematic sociological concept, to be sure, but with some value for parent-child relationships).
So, I agree that a 17/18 year old should not act on his impulsive feelings for a 15 year old, but I do not agree that it is unnatural as a social or biological process.