This blog post will have a lot of spoilers about the first game while speculating about Mary Skelter 2, so close now if you haven't played it!
Jack is still in the game, but his role is very different. Alice goes insane from Blood Skelter and nearly kills Jack, Tsuu, and Mermaid - Jack survives, but becomes a Nightmare. If you recall from the first game, Mermaid was dead before the game started. Jack was told that she had succumbed to Blood Skelter and had to be put down (maybe she took Tsuu down with her?). So, a simple prequel isn't a possibility. It's most likely an alternate timeline, and/or an alternate Jail. The trailer for the game also contains phrases like "A Jail that is different in every way" to bolster that hypothesis, but we'll have to wait for more info to know for sure!
Another thing I'm concerned about is that the game bills itself as a 'Mystery Solving RPG', but the basic setting (inside Jail, in an organization called Dawn that wants to escape, etc.) are the same as the first game, so the same mystery won't do, we need a new one! Hopefully the focus is a bit different this time than on just solving the mystery of what Jail is and what blood type girls are, since we already know.
Mary Skelter 2 is scheduled for release in Japan for PS4 on June 28th, 2018. While no English release has been announced yet, it's surely just a matter of time and I wouldn't worry about it. You can watch the teaser and OP here!