The case still has the feel to me of being committed by at least 2 people. The perp who drove him away from the school and the one who disposed of him IMO.
I still believe Kyron was told to look around a bit and be back out to the car a bit after 9. He knew he wouldn't be going on the tour with his group IMO, walked right past his class room door and off to that electrical project. He wasn't about to miss it.
I think that is why LE have put out this flyer. If Kyron was at the south entrance/exit, he would have had to walk or run down that stretch of building to the parking lot. If the bell had rung as Jules speculated, who would have seen him, considering there are no normal windows to look out of on that side of the school. The one window above the south entrance ~ you'd have to be standing there and look straight down to see him, eh?