I'm having fun reading this.
It's just like a slasher film.

The killer seems to be focusing on Trixie and Sunset, not really caring to wander off and try to kill anybody else. It could be he doesn't even know that there are other people in the school (though that is unlikely). It may be he's aiming for them because they're the only ones who know there's a murderer in the school, as the killer moved Poindexer's body before anybody else other than the two girls found it. Even though others will eventually discover the front doors are locked, he'll have the advantage of surprise. Mind games

At least they're thinking correctly about phones. I highly doubt it'd be feasible for the landline wires to be snipped, though. So far as I know, a building like a school will usually have a couple of redundancies for multiple phone lines.