Akashic Records of the Bastard Magical Instructor

Sisti attends a magical academy to hone her skills in the magical arts, and hopes to solve the mystery of the enigmatic Sky Castle. After the retirement of her favorite teacher, the replacement, Glen, turns out to be a tardy, lazy, seemingly incompetent bastard of an instructor. How was it that Glen was hand-picked by the best magician in the academy!? (Source: Tribuo Scans)