That mystery, in short, is the motor which keeps the film moving. And it's unfortunate that it runs out of gas a little too fast, because up to the third act there is actually a lot of fun to be had.
Bad Kids of Crestview Academy is the sequel to Matthew Spradlin's 2012 film Bad Kids Go To Hell. Both films are based on graphic novels, take place in an exaggerated reality, and follow the same set-up: some obnoxious posh students get detained for a day, then they suddenly find themselves unable to leave the school, and then they start dying in sticky accidents, if they're not being murdered outright. As long as the proceedings stay witty, thrilling and tongue-in-cheek satirical, it's a formula which can be very entertaining.
And Bad Kids of Crestview Academy has a strong point over its predecessor: a protagonist to root for. One of the students has been pulling strings to get in this specific detainment group, because it's an opportunity to get all the suspects and witnesses to the death of her sister in one place.
For two-thirds of the film this works fine, in the beginning as a potboiler and later even as a slasher, when the over-the-top deaths start to occur. The technical merits are good, the acting mostly competent, with both Sammi Hanratty and Colby Arps actually being great as the leads who try to solve the mystery.
But in the third act the film crashes and burns, badly, when it seems to try and change into Sin City 3. Satire gets replaced with unfunny caricatures, where everyone sneers about their schemes (and would probably be twirling their moustaches if they had any). Unfortunately the plot of the earlier film suddenly becomes important as well, which is a pity as the new film was doing fine as a stand-alone so far.
In the end, Bad Kids of Crestview Academy squanders its mystery on telling a joke it has told several times already, and all tension leaves the film, making it hard to care for the remaining survivors.
Your mileage may vary and fans of the first film will definitely find much to like here. But for me. Bad Kids of Crestview Academy reminds me of a party where everybody is drinking too much. At some point during the evening, everything clicks, everyone is awesome, and all jokes are funny. But check a few hours later and it's mostly dumb embarrassment, where even the best find it hard to remain standing. And it's a pity, for when the party was at that point earlier in the evening, it all looked so good...
(Bad Kids of Crestview Academy will be released in selected US theatres from 13th of January onward.)