I'm Giving the Disgraced Noble Lady I Rescued a Crash Course in Naughtiness 8 (manga)

Powerful sorcerer Allen and his crew arrive at his old stomping grounds, the Athena Magical Academy, only to find it in the throes of a fierce battle for campus dominance—and Charlotte’s little sister Natalia at the very center of it all! While Allen joins the fray and kicks things up by several naughty notches, Charlotte goes undercover as his assistant to get closer to Natalia. But just as their plan to bring peace to the academy and reunite the two sisters hits a critical juncture, Natalia vanishes into thin air!