I'm at a loss as to why the action of this is not set up in high school. Putting it in middle school is of no benefit to the plot, other than by significantly increasing the creepy factor. From the chapters we've seen, all would make just as much sense, if they skipped directly to y1 HS vs y1 middle school. Maybe even more, since most 12-year olds are not really jumping on the romance train immediately. It would also be way more plausible for an adult to be somewhat attracted to 16-years old, rather than to 12-years old.
In all honesty, that's probably the way I'll keep reading it. I'm guessing there is probably a significant part of the story when she is in high school, somewhere before the end, when she's in her 20s, but I'll cross that bridge when I get there.