But I want to do something a little different: give my prediction of who the murderer is, based less on anything that's happening in the show, and more on pattern recognition for how murder mystery shows work. There aren't spoilers here per se -- unless I'm right, that is.
Anyway, my guess for the murderer is Mr. Shapiro, the science teacher.
Well, my first principle is that the murderer will not be any of the main characters. In a whodunnit type series, whose premise is alternating who among the main crew suspicion is cast upon, it's really hard to finally land on any of the main players without it feeling a little anti-climatic.
At the same time, the murderer has to be someone whose name has been mentioned and who has already been confirmed to be on scene when the murder happened. You can't just say "Surprise! There was an unknown stranger hiding in the closet the whole time!"
So ideally, you want someone who has been mentioned and confirmed to be present, but is otherwise forgettable. He isn't part of the group that comes to mind in the list of suspects, but once he is revealed as the killer you can't claim that the show is cheating.
Mr. Shapiro has, of course, been named. And though we haven't seen his face, it was confirmed that he was at the titular afterparty (having sex with someone in one of the spare bedrooms). It's been treated as a throwaway gag (he was also walked in on having sex with someone in one of the classrooms during the reunion itself), but it suffices to establish his presence at the scene of the crime such that he could be a suspect. That he hasn't been seen since -- that nobody seems to remember he was present at all such that he should be among the group of suspects the detective is interviewing -- only points further in his favor as the ultimate killer.
We don't have a motive yet, but that can be established later. Again, I'm just working off genre conventions here.
Anyway, I just wanted that prediction on paper because if it's correct I claim the right to call myself a legend. But again, it's a very good, very funny show and you all should watch it.