I'm thinking it is one of the two people working on Richmond's campaign. I'm way stoked for the finale.
Me too. But they said a shocking finale. Neither of those people would shock me. The teacher would be interesting to me because he's been an afterthought for about five episodes now and if I recall correctly was never truly exonerated from the crime. Yet we know he had feelings of some sort for the girl, had the chemicals in his house that were found on the body, and was with her earlier that evening. Yes, his original suspicious activity had to do with smuggling the girl to Canada but that doesn't mean he didn't do this.
Who's more likely is the current mayor. He has motive (ruin Campbell's campaign by having the girl found in one of his campaign cars) and the power and money, primarily to coarse the Indian casino peeps, jurisdictional differences notwithstanding.
Also, the parents, if you remember correctly, were "camping" the weekend of her murder. Perhaps some Shakespearean chain of events could have brought one of them to do it.
All that said, I think it's Billy Campbell's girlfriend like you mentioned. She had seen the teeny bopper in pics with him, perhaps had followed him or her the night of the murder and after their rendez-vous, assuming they did have one, confronted the girl, and then murdered her in a jealous rage. Pretty simple. Pretty reasonable.