Rhonda's abandonment of the other children to the zombies. On first viewing this appears to be an act of sheer spite. But then, remember what Macey said earlier: the elevator could only hold three people safely. With Sara being dragged off, there were four children left. While Marcy was probably lying, Rhonda probably didn't realize that. So, she naturally assumed they'd push her out and leave her to die.
A lot of stuff. In particular, look carefully at the chocolate bar that Steven gives Sam, and which character is wearing a class ring.
Why was Laurie so embarassed about the Red Riding Hood costume and why did her sister and friends laugh when they saw her in it, despite wearing similar costumes? They're all werewolves. Apparently it's also tradition for a werewolf without her first kill to dress as Red Riding Hood in order to nab it.
Even if Macey and her friends hadn't been killed by the school bus kids, they probably would have died later anyway when they went home and ate the candy Mr. Wilkins gave them.
Fridge Brilliance: Not necessarily. Mr. Wilkins made note at the beginning of the film of the "rules" to keep one safe on Halloween. Charlie forgot the first one, but it's possible that Macey and Co.'s parents would have checked their candy. Wilkins candy would only be dangerous to someone who broke the rules.