Old man: "The quality of this school ... will come crumbling down."
The quality of a school isn't about which students attend, but the faculty and facilities and programs.
What he's actually worried about is the school's reputation. But here he is being far too pessimistic or he doesn't...
More like nobody would have noticed otherwise. And the protagonist didn't throw a fit over it, keeping it even more off the radar. So it's not right to judge the entire industry on this one particular case.
That's not Kamo's grandma, it's Granny Kamo from the next town over that isn't related. But they know each other since Kamo-kun sometimes gets her mail by mistake. See chapter 10.
Starts with a similar premise as "Climbing to The Top of the Esper Academy" by EBA. It's hentai though, so content filter settings might block it if they're enabled for you.