Heim appears as early as chapter 2, though it seems like his design wasn't fully pinned down at that point; he's the guy who takes the eyes of the succubus after the raid that makes Aine swear vengeance on adventurers.
There's never been any explicit statement that Aine and Nacht are in high school, it's just sort of left implied by the uniforms and the archetypal isekai story involving someone in that age range being transported to another world. There are also things like chapter 9 establishing that Aine (and probably Nacht, judging by her reaction) is a virgin, the joke on the sign at the entrance to the dungeon implying compensated dating, and the general naivete Aine displays (see the... interesting things that the succubus was teaching her).
But then there are things that seem to imply that they're actually older, like Aine saying that she's seen the inside of the, uh, "Medusa Excursion Vehicle" more times than she's seen her own parents' faces, and the revelation in this chapter that she's played the original Resident Evil. They add up to some things that make you go full thinking emoji and start to wonder if the implied premise of the isekai parody setup is a red herring and if Aine and Nacht are actually a pair of middle-aged fujo NEETs who still inexplicably wear school uniforms even though neither of them is going to see the south side of 30 ever again.