Yoru wo Utau Kodomotachi

From Aerandria Scans:

“Those who meet the owner of the secret mansion never return to their world again…”
There are rumours spreading around Kunuyuki's school which say that students mysteriously disappear one after another from the campus, and that there is a secret door hidden somewhere. This time the missing person is Kuniyuki’s best friend - Kashiwagi. Are the rumors really true? Kuniyuki is angry and has no idea what to do about his friend, when he meets a strange, but very pretty girl called Towa. She says she is the guardian of the “outcasts,” those who were taken to the “mansion,” but what does that really mean...?!