Wow; this sounds insane. Too bad they don't make invisible electric fence! It is creepy that a 15 year old girl is hanging around your house.. doesn't she have a social life? lol.
I think that cameras are a great idea. My husband ordered some out of a catalog for us, and they were very inexpensive; I am pretty sure, around $50. I will ask him later; maybe they have a website. Heh.. I would be inclined to whip out my cell phone and photograph them following behind in the car.. or.. (and this isn't what I recommend) slam on my brakes while they followed. But that's just me