TL;DR I think everyone except the main human girl is actually monsters
Im glad I found another horror comedy that has that true horror vibe while being funny, I do have a theory that "humans are the actual monsters but the monsters mimicked them to take advantage of how easy human life is" mainly due to the slender man teacher, him / her saying to replace the class with monsters would be cool but I think the truth is that the whole school already is monsters and he just hasnt realised since he had no clue about other monsters
he didnt even know about the teacher getting murdered, which means he had no clue our main character is a monster so by that we know monsters cant identify other monsters and its highly likely we will get more monsters [my suspicion is the girl into older men is the monster even tho i think both are monsters]
this may seem a bit far fetched but just think about it logically our main girl got caught by a socially awkward loser and was killing in an area that often gets visited so we know these monsters arent good at hiding their killing [or atleast she isnt since she does admit to getting caught multiple times then just killing the witness]
the main character didnt know monsters existed before so its clear monsters arent believed to be real which is highly improbable with how easy it would be to catch them so my idea is that everyone is a monster so their constantly covering for eachother and is also why our human character is so socially awkward. she isnt trying to cover up anything since shes the only one that has nothing to cover up, her grandma is weird because their all human not a single one is a monster so they dont have to hide who they are