A few things...
1. The timeline is really too tight for this to have happened in that way. LE seems to think that the time between Abby's picture being taken and the girls' deaths as really quickly-less than 2 hours, if I am remembering correctly. Taking them into town, driving around, coming back, etc...not a whole lot of time. Abby's picture was taken a little after 2pm and by 3:30 (before that, really) someone was looking for them.
2. It wouldn't explain the recorded convo about the man behind them. We don't know if they really used the word "creepy" or not, but they did seem to comment on his presence and not in a "let's hang out with him" kind of way. I don't think they'd have had that convo and then gotten into the car with him in what amounts of a few minutes later.
3. It wouldn't explain why Libby started videotaping him on the bridge. Why turn it on and secretly film him and then agree to go ride around with him? (Unless the whole videotaping of him was an accident and she hadn't meant to.)
The scenario would explain a few things-why nobody heard them scream and the phone "pinging around town". It might also explain how they got across the creek without wading through it. I think there are other, more probable explanations for those things though: they didn't scream because they were too afraid, the phone was simply bouncing off towers because the towers were full, and they were either ordered across the creek, crossed back over the bridge (and were accosted THERE), or they panicked and ran.
It's not a bad idea, though. I don't want you to think that I'm pooh-poohing it. When the crime first happened, that's what I thought had happened. I thought the girls had met up with some peers close to their age and that they'd either been bullied or hazed and it had culminated in their deaths. As the facts started coming out, though, my theory no longer made sense.