Akashic Records of Bastard Magic Instructor

Sistina attends a magical academy to hone her skills in the magical arts, hoping 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, and seemingly incompetent bastard of an instructor. How is it that Glen was hand-picked by the best magician in the academy?!

(Source: Tribuo Scans)