@Petnavis
That's some long explanation trying to say you're not creeped out by a depiction of a sexualized middle schooler.
Thing is, if you know who Norio-sensei is,
her subject matter tends to lean towards primary schoolers. I think they settled on the the middle school setting as the middle ground. So no, the Yamada design is not a college-aged girl with her age changed to 14, it's likely that the age was increased instead. Yamada wasn't this well-endowed in the earlier chapters, but I guess Norio-sensei ended up liking her too much (if you could tell by her twitter handle and pixiv profile pic, she probably likes cute dorky gluttons) that she gave her upgrades.
Also, I've had really tall female classmates during my middle school years. It's not unusual. I bet Norio-sensei was one herself if it wasn't a friend or a classmate.