If the author decides to do something like where one of the three girls gets molested or almost raped by a third party, I will be dropping it cause these types of mangas tend to do that often.
Chapter 6, Amajiki states that she's never been in contact with a man all her life. So I think it's safe to assume that she's fatherless (either through abandonment or death), and has no male siblings (but potential female siblings). It starts to make sense why she's so curious about sensei and why she does the things she does to him.
Also, is Sensei the only male teacher in the facility? I've never noticed but it seems like he is.
i usually avoid manga with the smut tag, but this one didn't have it, but i think it now deserves it. problem is, now, i'm a bit invested in these characters, so how am i supposed to stop reading it. that's cheating, mangaka, lol