The Chant of Yalva by Garth Notely

A modern English school collides with a thousand-year-old monastery from a parallel universe. Can one boy’s quest to rescue his abducted girlfriend topple a dystopian cosmos? It seems like a pleasant English school in a small country town. But something’s going on. People are disappearing. Every new moon a child is taken. And it’s all because the County Council built the new school on the site of the ancient Monastery of Darkmere Fen where, in the year 1005, Abbot Aedan created his own universe with its own rules, where he is God and everyone must worship him.