This story feels like it comes after the events of Blood Covered, but it seems like it might be the events of a timeloop, which is one of the game's endings that don't give you a "bad end" song when you achieve it.
My basis for that is the fact that we see Seiko as a ghost, Mayu's uniform in the wall where she was thrown at inhuman speed (which is different from the original work's death, as her uniform seemed to disintegrate in the other one), and Yui-sensei is not pinned down and unable to move. Aside from that, Yoshiki and Ayumi are also not shown in this manga, probably because they already died in this loop, or are temporarily back in their dimension (when Yuki takes them both back and Ayumi decides to return and save everyone).
But enough ranting, this manga was certainly good, albeit rushed (probably because the main plot is the same, albeit new characters are the stars this time) and it was a good read, worthy of a 7 rating with a cherry on top.