Having re-read the current featured series (and remembered a few others), I've once again come to realize that as cute as Itou Hachi's art is, as much as she's known for her loli, clearly her true talent and passion lies in drawing trains.
Did not see that twist coming... Well, no, that's a lie, I did have that thought somewhere back in my mind, and I fully expect it to be the usual trickery, although the situation really looked a lot worse than I may have expected. :D
Seifuku no Vampiress Lord by Matsumoto Tomoki. A male vampire converts a teenage girl only to find she has zero interest in him because she's a lesbian who already has a girlfriend. https://dynasty-scans.com/series/seifuku_no_vampiress_lord
As great as the series is, no, she does not already have a girlfriend, the (accidental) confession comes quite a bit later, and she is also rather surprised by the revelation that her taste in blood also indicates her orientation. She does accept the fact quite easily, though, and from then on...
I mean, Mei's discomfort isn't much of an indication in either way, seeing how it has little to do with the teacher being a man and all about him simply being a horrible person regardless of gender.