First off sorry for how vague this is probably gonna be. I honestly can't remember if this was an anime or a manga. The male teacher is a sort of a side character, I think, to the main plot (and I can't even remember the main plot) and one of his female students starts ends up staying at his place for some reason or other (I feel like maybe there's a scene where she calls him in the rain?). I think she tries to make a move on him because I remember him stressing out over being too old for her but also kind of liking her and being conflicted over that. Also conflicted over having her stay at his place, like he actively wants her out but also kind of doesn't.

If it helps at all I just started watching the Doukyuusei movie and for some reason Hara-sen reminded me of it? Maybe it wasn't a female student and I'm just remembering Hara-sen side story? Except that's not on any of my lists so maybe I didn't read it when I read Doukyuusei.