Lonely Castle in the Mirror by Mizuki Tsujimura

Seven students are avoiding going to school, hiding in their darkened bedrooms, unable to face their family and friends, until the moment they discover a portal into another world that offers a temporary escape from their stressful lives. Passing through a glowing mirror, they gather in a magnificent castle that becomes their playground and refuge during school hours. The students are tasked with locating a key, hidden somewhere in the castle, that will allow whoever finds it to be granted one wish. At this moment, the castle will vanish, along with all memories they may have of their adventure. If they fail to leave the castle… Read more.