So from what i recall, its was a long time since i read the Novel, but in this World, Gods, Monster and Dungeons exist...
Monster are formed from idea/imagination Power, and so can take various form and are difficult to dealt with, so "people" (can’t recall if some Gods was also involved in the following.....) created the Dungeons as a Trap/Jail, that hold All the Idea/Iamgination Power. And now, in Modern Time, to avoid having the Idea/Imagination Power overflow from the Dungeon, and begin to spawn Monster outside, the School/Academy is formed to delve it, to spend the Power by forcing it to respawn Monster killed by the Students.
Several "Rules" about the School and the Dungeon:
-First, when killing Monster, the Student absorb a little of the Power to grant them Strenght (in the form of Class, Level and Skill)
-When you die inside the Dungeon, for the First time, you left a part of your Soul in the form of a Crystal as offering for the Dungeon.
-When you die in the Dungeon, you respawn/reapper outside it, but you are unconscious/mindless until the Door of the Dungeon close, and you lose all memory of the Delving Run. (If you don’t die you recall everything, but if you die you can’t learn from your error....[possibly making you stuck inside a loop of: Dying, forgetting, retrying, dying....])
-The concentration of Power in the School ground is Minimal, enough to keep some Strenght from the Dungeon, but not enough to use Skill.
Now some "Spoiler" that happen inside the Novel/manga:
Because Monster are formed from Idea/Imagination Power, the monster recently (~20 years ago...) changed from Japanese Yokai, to the Western Goblin and Orc (the rapist version...)
In addition of the Sex/Rapist Monster, with the Stress from killing and being near death, and the "arrogance" from gaining "Superpower", the school allow the Students to "enslave" other students and also no sexual act done inside the School and Dungeon, will be considered non-consensual.....
Class are organised in "Categories" (Swordman, Mage, Officier/Support, and even NEET...) and each Categories has "types" (some sort of Class line; like One-handed Swordman, Two-handed Swordman, Dual-Sword User...). Also to level up a Class you need to either do act that are related to the Class, or kill Monster. And you can change/upgrade your Class only once you maxed out your level. For example, the Teacher has a Sex-type NEET-Category Class (Cumdumpster, Whore, or a similar name...) and to level it to change Class (because she isn’t strong enough to kill Monster....), she need to be used like a Meat Urinal for cum....(in the novel, you often see her complaining about people doing something to her Students, before she explained thing to the Students, only to have the other party just pinch her Nipple, unleashing the Niagara Fall inside her panties, before being fucked, forgetting her argument, and going :"I love Dick, i love Dick....".)
Because in old time, when the Idea/Imagination Power was present in the World, Sexual relationship was used in Magic Ritual, now at School when a Man has sex with a Woman (or more commonly called a Female in this story...), they can Dominate them, if they do have enough sex/creampie with her (the condition depend on the Strenght of the Man versus the Strenght of the Woman, in term of Class and Levels, with a Man being a High-Level enough, that he can Dominate a Woman with a Single creampie....), and the Dominated Woman can’t resist any command from the Man, is easily aroused (just a stare is enough to flood their panties....), and she feel the double or triple pleasure when having sex with the person Dominating her....
Also, when losing your Soul/Crystal inside the Dungeon, you end up Infertile, and other thing... (from the known side-effect...). Also, about the NTR, Touma/the MC doesn’t have any of his Women stolen, he does stole some women, sometime....
NTR, happen only in alt POV, from other people in the School Class, or School Grade...
Also Touma, is DENSE!!!!!!, one time in the Dungeon, he used his Lightning/Thunder power to exterminate Monster (can’t recall if that was to summon a Special Boss, or to save some Classmate/Lovers....) and he goes on a Rampage....
Once out of the Dungeon, he hear peoples talking, about a "Thunderstorm" in the Dungeon, that erased "everything" and left the ground scorched, on the Floor he was....
And Touma's reaction is :"Hm, i was here, but i didn’t see this "Thunderstorm", lucky i wasn’t near it, it seem many peoples died....". (You can see the LEVEL of DENSITY....)