King’s Proposal, Vol. 3 (light novel)

After receiving word about her engagement which she knew nothing about, Ruri heads back home to the Fuyajoh household. Mushiki, who is worried about his sister, seeks out the Fuyajoh's family head and discovers they are the principal of an all-girls' mage training institute! Unfortunately, no boys are allowed so he infiltrates the school as Saika Kuozaki, adopting the role of a special lecturer. But will he be able to stop Ruri's marriage in time?!