"This could be true, but how do you explain their cell phones working even when they are turned off? Or the one cell phone turning itself on and sending a text message to the other phone, while they were sitting on the table during the interview with the school principal and police?"

>>Not trying to explain it... but I am trying to explain that whomever is doing this... will escalate their program when it appears that the current tactic is no longer scaring his/her/their targets.

As for how, I will speculate that someone is 'hacking' the cell phones in some way. I am a firm believer in the idea that an intelligent person can make nearly anything happen if they set their mind to it. Some idiot found a way to hijack their phones and it using it to terrorize them.

Go lowtech to catch him... simply hide in the bushes and wait.