How many loose ends would he be likely to leave if he had all of his things packed in a bag (or on his body), he had a way of leaving in a hurry if necessary, and he knew he hadn't been seen by anyone who knew him (with the possible exception of a friend/accomplice), and killed the witnesses. I think it's more likely that he saw the girls and did plan to do what he did without them doing or seeing anything to make him feel like it was necessary.
I was mainly trying to make the point that there may be other options than that they had arranged to meet and that he just happened to go there looking for girls while they were there. I don't believe the girls went there to meet anyone, and I'm not totally sold on the idea of luck. If nothing else, he was probably familiar with the place, knew there was no school that day, and was sure that sooner or later there'd be someone he'd be able to overpower without witnesses. I'm always open to ideas, although even I don't put much faith in a lot of my own.
