"She's old, but she looks young for a person with TRANSFORMATION magic that allows her to even completely imitate my own Youthful Appearance... Surely this means she's an EROFU!"
I think you're just reaaaally hoping for an Erofu there buddy.
But even if she isn't one, she could turn herself into one for you.
Though because the MC is saying this guess out loud, I'm assuming that the author is just going to go that route, it is difficult for Isekai to resist the urge to have an Erofu.
And the kid is in for some rough lessons as the teacher gets him to Dominate her harder through strict training.