I Kept Pressing the 100-Million-Year Button and Came Out on Top, Vol. 5 (light novel) by Syuichi Tsukishima
Though Allen managed to protect his friends from harm when the Black Organization attacked Thousand Blade Academy, the same can’t be said for his school, which now lies in ruins. While their campus undergoes repairs, the students of Thousand Blade are transferred elsewhere to continue their education…and Allen ends up at an all-girls school! Later on, he and his friends are sent abroad to liberate Daglio, the Land of Sunshine, from the tyranny of the Black Organization. But this will be no easy task with one of the overwhelmingly powerful Thirteen Oracle Knights facing off against them. Can the former Reject Swordsman awaken to his true power and set things right?