Lonely Castle in the Mirror (Manga) Vol. 4 (manga)

The kids had promised each other that they’d all attend school on January 10th, but when the day finally comes, the seven kids fail to meet up. With the key still missing, the month of Ookami-sama’s deadline finally arrives. The kids decide to treasure the time they have left, but on their final day in the castle, something shocking happens at the very last moment!