The Invincible Little Lady: Volume 4 by Chatsufusa

Mary discovers a secret room built under the Academy. When she and Prince Reifus go to investigate it, they discover a box containing a circlet. At the behest of a mysterious voice, Reifus puts it on, causing him to transform from a handsome prince...into a beautiful princess?! Feeling responsible for her involvement in the incident, Mary tries to resolve the situation—but fixing this won’t be easy!