He must have planned to do this in advance since he brought the box cutter and several change of clothes. The way it reads, the teacher talking about Tennessee set him off, but he wouldn't have known she was going to discuss that after school or that she would even ask him to stay after school. It makes you wonder if there was some other exchange prior that had him in the mind set to kill her, and the Tennessee thing was the last straw.
I also can't believe no one noticed him in a ski mask and dragging a recycling barrel through the school,...I mean, good God. Wouldn't someone notice that??
So very disturbing and sad. If he had no prior problems or crimes, he sure sounds like he was a ticking time bomb, waiting to explode.