I was thinking the same. Man got horny from one moment to anotherYamamoto Souichirou seems like he's getting antsy to lewd out his characters. It didn't really have a place for Tagaki, but he's getting fairly suggestive with Urushi lately and now we have to go to the onsen regularly to advance Tsubaki's story line.
That was my initial thought as well, but it seems like she's just as curious about Tsubaki's change in demeanor.I wonder if Benisumomo is actually the letter writing "boy".
I'm also suspicious of the teacher(s), though it could also be them monitoring the situation. Regarding it ending "soon," there's also the chance that the manga could "change premise"(from Tsubaki wanting to know about boys -> secret meetings/dates) and stick with that instead.So is either the teacher or the manga is ending "soon". Considering this mangaka really loves to keep the status quo on all of his manga, he only do sudden development near the end.