Volumes 10, 12 and 13 contain the extra story "Usui-kun Haou e no Michi (Mousou)" (碓君 覇王への道（妄想）), #1-3.
Volumes 11 and 12 contain the gaiden "Maken-Ki! Kai!!," #1-3
Volume 14 contains the extra story "Espada no Karei naru Harem-dou" (エスパダの華麗なるハーレム道), #1-2.
Maken-Ki! Kai!! (マケン姫っ! 開!!); Age Premium
Set years before the main story, this series follows Minori, Aki, and the other teachers during their days as students at Tenbi.
Gakuen Dragon Slayer (学園ドラゴンスレイヤー); Dragon Age 2015-11
A school is transported into a fantasy world that is at war with dragons, and everyone in the school is given a powerful dragon slaying sword, and told the only way to get home is to help win the war. But there is a catch, to use the swords power you must be...a virgin!