If these girls are in the 9th grade and are 14, then that is one very creepy teacher.
I think they're supposed to be 16-year old juniors. The one who renovated the barn loft (Spencer?) said that her parents had said she could live out there her junior year. Now, I don't know who lets their daughter move out to the barn before she graduates from high school, but I can't imagine the callousness of parents who tell their daughter that she can renovate the barn and live there, then let the sister tell her that they've changed their minds and sister's going to be living there instead.
but besides the way the girls looked (Emily's supposed to be a redhead, Spencer and Melissa are blondes, and Hanna has auburn hair) and how much time has passed between Alison's disappearance and when the notes start appearing (book is 4 years, show is 1), everything wasn't that inconsistent.
There was some oddness about how they talked about how long it's been. It almost seemed like they changed their mind partway through. For instance, the reporter said that the family confirmed that the gazebo was being built "the summer that Allison disappeared" rather than "last summer, when Allison disappeared"...
I figured it can't be any of the girls because they all got text messages and why would A send a text message to herself. It can't be Aliison because she's dead unless ghosts can send texts to people.
The girl that didn't get a text would be under suspicion from the other girls, so if "-A" has any intelligence, she'll send herself texts and arrange to be spotted getting a disturbing note. As for Allison, I was wondering whether she could possibly have faked her body. I mean, it seems ridiculous that a 15-year-old could arrange to hide a body and make it look like hers, but this is a show that asks us to believe that these "girls" are 16... In that context, I think there's room to question whether the police actually checked dental records, etc., or just went off "body, girl, right size-ish, right clothes, in Allison's backyard = Allison".