The Knight Captain Is the New Princess-To-Be Vol. 2 (manga)
In order to support Prince Leo, Chris agrees to officially take his hand in marriage. But the other nobles are not fans of the union, and to mollify them Chris gets sent to a boarding school to become more “ladylike.” Unfortunately for her, her fellow classmates are young ladies intent on marrying the prince themselves! Chris might be hot stuff on the battlefield, but is she prepared for this kind of warfare?!