I've been casually keeping up with the development of this game for a while, but I'm always blown away by the many features the developer adds with each update. I'm actually not surprised about the surprising popularity of the game with preteen girls because well, I was a preteen girl myself (though not a yandere preteen/teen) and I've certainly had thoughts of getting back at other girls who'd bullied me then (though I never acted on them) - I'd have appreciated having such a game in those times as my outlet for frustration. (I used RPGs and grinded like mad instead.)
I actually wonder when the game will be finished, though. The latest build says the game is only 5% complete. (Maybe I'll have my own preteen girl when it gets finished? Haha.)