Hana no Kishi

From Taciturnity Scans:

Ever since the day Ran Kurono's older brother was killed while protecting Sei Ohtori, she has dedicated her skills and life to one day becoming the First Knight of the next head of the Ohtori family, even concealing the fact that she's a woman forever. For generations the Kurono family has served the Ohtori family as their First Knight, but 11 other lower knights with ill intentions of using the Ohtori family's authority vie for that title. Not only that, but Sei is being tested for being suitable for the title of head of the family. Upon entering Saint Locks Academy, the days of both Ran and Sei's trials begin, all the while being helped by Sei's untidy and unrefined fiance, Ibara Tennou.