The Principal's candy is only poisonous if you eat a lot of it.This occurred to me after I realized that if there was a mass death-by-poison in the neighborhood, sooner or later, it would be traced back to the source. So I came up with the theory that if kids follow the rules of Halloween, and only take one, as the sign on the bowl instructs, they'll be fine.
Sam is behind it all.In all of the segments Sam is usually shown observing it. Which means that Sam might have orchestrated the events that have been happening in the movie and unleashed the supernatuarl creatures in the movie to punish those who don't respect the tradition of Halloween.
Rhonda was a real witch.Or at least training to be one. That would explain her being nonplussed about the other kids' deaths or seeing Sam.