Aria is A

So playing around with the theory that Minnie is working with A...

This episode establishes that she communicates with older people pretty regularly. And can even ask favors from them (she asked him to go trick or treating). We don't know the reason yet, but so far it seems like this is a healing kind of role playing for parents who lost their kids and she acts like their child. If that's true, then a parent who lost their kid to bullying (like Angela Water's family) might use Minnie's "services" to get revenge on the women who bullied their kid to death. It also helps that these girls "proved" to be bullies themselves when they screened the video of Karen. Anyways. That's just a possible reason for the why.

In the Halloween episode, there were a couple of things highlighting Minnie. Namely arrows. The first one might be a bit of a stretch, but hang in there.

An arrow next to her pointing up.

The three arrows in pointing at Minnie. Also, three is important because the third person dies in this episode.

This one is more obvious

Then the whole altercation at the party begins with Tyler coming on to Minnie. He's the third person to die because of A (Davie and the janitor being the first two) and he has a flag with the number 3 point at him during the altercation scene.

When Noa says she's get Shawn to beat him up, Minnie says "Or I'll just do it." (Or have someone do it for her???)

Afterwards, when they bring up A and how he hasn't shown up, one of them jokes that A might've been trick or treating. Minnie was the only one that went trick or treating that night.