Engage Knight story by Chika Isaka and Mayu Kasugi (2 volumes- complete)
Tiaris enters the Noble Course of Grunwald Senior Imperial Academy to fill in for her sickly twin brother. No, she didn't pretend as a boy but she actually volunteered to be one and the other girl flatly pointed to her that she's going to be found out. Gender bender aborted. Aside from being a substitute, she's also given the task of looking knights for their kingdom from the students of the Knights Course. The cover and the summary of knights coming after her made me want to read this. The "engage" part is an original idea too but only 3 chapters are available so far. I'm inspired again to create the tag "knight" for other similarly themed titles.