コウ (kou) is the on-yomi used in compounds like 学校 (がっこう, gakkou: school), 校長 (こうちょう, kouchou: school master), and 小学校 (しょうがっこう, shougakkou: primary school).
This kanji is almost always used in compounds and rarely appears alone.
Etymology: Tree (木) + crossing (交) → place where teachers and students meet and study → school