We where very short of that when they where on bunny suits
.also that sensei is suspicious,why I think she knows about them and will exploit them
I kinda think Tachibana is on MC and the Twins' side, ultimately.
If I remember correctly, she seems to have the inside scoop on MC's "past" as it were (his being bullied in middle school for his intelligence and lack of social acumen at the time), and once MC helped with the one twin's truancy, it seems Tachibana has mellowed out in that vein and is helping MC out--and by extension the twins--from the background.
She might still exploit the three of them to some degree, but not to any nefarious ends, unless everything about her has been a fakeout up to this point.