Akashic Records of Bastard Magic Instructor light novel vol.1

Lumia and Sisti are mages-in-training at a prestigious magical academy where they hope to be taught by the best of the best. However, when their favorite instructor suddenly retires, his replacement turns out to be a total jerk — he’s idle, incompetent, and always late! Can Lumia help uncover their new teacher’s true potential — and can Sisti still learn magic and unravel the secrets of the mysterious Sky Castle with such a terrible mentor as her guide?