Anna and Miku's age is right in the title of the manga!
But yeah, Japan does 1-6, 7-9, and 10-12; unlike the US it seems a universal pattern. So Anna's in 9th grade, far enough in to be 15 and not 14.
"So, yeah, Ema is the most morally grounded character in the story. (Wanders off cackling at the very thought.)"
Ergh. She keeps perving on her 15 year old ward who has nowhere else to go. (Well, I guess she could call her parents... not sure it was clear why she's with Ema, apart from "their convenience" and Anna being happy to get away from their strictness.)
Miku is making a running leap for the Moral Event Horizon but at least she's a teenaged peer who doesn't live with her target. Maybe she has messed up ideas about romance and consent.