The manga of the popular Light Novel series of the same name “Roku de Nashi Majutsu Koushi to Kinki Kyouten” / “Akashic Records of Bastard Magic 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!?
Also, if you’re interested in mangas with a magic academy premise, be sure to try out Academy’s Undercover Professor. It has an overpowered mc who got isekai’d into a world of magic and somehow ends up taking the identity of a newly hired professor at the magic academy.