My head cannon (if that's the right phrase) for this game, is that the tentacle monster was terrorizing several scools in [REDACTED] city. So the local govament laid a trap to keep it from having it's way with the youngs girls of the country. Once it was in their trap, they hired many youthful women to help bait it into staying in there until they find a way to kill or contain it.
This would explain several things, like why all the girls are 18+, why none of them have any underware on, why they enjoy the tentecles and why none of them are going to class.
I also have a suggestion for the cosmetic unlockables. When you buy them, you get a chance slider that starts with a range of 0% to 10%. Buying the cosmetic again will increase the max by 10% each time until it reaches 100%, the cost should also go up by 10% each time. Conflicting cosmetics, such as the fox girl and cat girl, when the combind totals of the group is greater then 100% increaseing one will reduse the others. A better way to explain it, is say a player gets both the fox and cat to 60% each, setting the fox to 60% and then setting the cat to 60% would reduse the fox down to 40%, keeping the combined chance to 100%.