Lonely Castle in the Mirror, Volume 1 (Lonely Castle in the Mirror #1-5)

Kokoro is a junior high school student - but after years of bullying, she's become so anxious that she no longer goes to school. One day, she's pulled through her full-length mirror into a castle in another dimension by a girl in a wolf mask. The mysterious girl tells Kokoro and six other kids that they will compete in a scavenger hunt. If one of them finds a key that unlocks a secret room, that person will get one wish granted. As long as they observe the rules of the world, they can come and go as they please. The seven kids gradually grow closer as they explore the castle, but the scavenger hunt is never far from anyone's mind. After all, only one can win their heart's desire.