After going through "Good Boy" on my replay, I have some thoughts I want to share regarding the school(s) we see in happy events. As
Apollo259 has pointed out, the school hallway looks the same as in "Den of the Mole Rat", viewed from a different angle.
In addition, in "Good Boy" we see what seems to be a version of the same classroom from "Turn Off the Lights" and "Den of the Mole Rat" - look at the objects above the chalkboard.
But there are differences as well. For one, the chalkboard text from the other two events is missing in "Good Boy". For another, we can see sunlight in "Good Boy", whereas that's something that supposedly never happens in the "Turn Off the Lights" classroom.
There are also indications that the "Good Boy" school is a real place in Sensei's world, rather than in another plane.
HOPE's angels send Sensei to a specific time and place in order to talk to teen Sara. So this is probably the high school that Sara really attended.
The name of the school is redacted, so it probably has some significance to Akira's past as well.
After talking to Sara, Sensei has a memory of Sekai, and she disappears right as we're shown the classroom, with a sunflower in a spotlight at the front of the room.
To me, the sunflower is standing in the place of the teacher, which suggests that this is a/the classroom where Sekai taught. So I think this is a school where Sekai was a teacher, Sara was a student, and there's a good chance that Akira was a student as well.
I've
posted before that the den of the mole rat seems to be a place that Pareidolia/Sekai have control over. I think Sekai's school has been recreated at the bottom of everything, either because it's a reflection of her mind or they have intentionally reconstructed it.
I think there's a good chance that the abandoned school that we keep seeing and which is apparently outside the barrier is also Sekai's school, since there's no reason to assume two different mysterious schools in happy events. I'm not sure whether it's the real school, the reproduction at the bottom of everything, or if it varies.
This school is also connected to Sara, since Sensei somehow wanders to the school after leaving Sara's apartment in "The Place She Falls Asleep at Night".
