～校
～こう · ～kou · JLPT N2 · noun
school
🏫 校 = 木 wood + 交 meet: where people meet to learn; 三校 = three schools
Put ～校 after a number to count schools: 一校 (いっこう), 二校 (にこう), 三校 (さんこう). The same 校 is in 学校 (gakkou, school) and 校長 (kouchou, principal), so the counter is just the word for school.