Lonely Castle in the Mirror by Mizuki Tsujimura

In a tranquil neighbourhood of Tokyo, seven teenagers wake to find their bedroom mirrors are shining. At a single touch, they are pulled from their lonely lives to a wondrous castle filled with winding stairways, watchful portraits and twinkling chandeliers. In this new sanctuary, they are confronted with a set of clues leading to a hidden room where one of them will be granted a wish. But there's a catch: if they don't leave the castle by five o'clock, they will be punished. 

As time passes, a devastating truth emerges: only those brave enough to share their stories will be saved.