As the school's key-holder, he is responsible for making sure all classrooms are locked every day.

key-holder 是由名词 key（钥匙）和 holder（持有者）组成的复合名词，字面意思是"钥匙持有者"。 holder 来自动词 hold（持有、握住）加上名词后缀 -er，表示"持有某物的人或器具"。 key-holder 指专门负责保管钥匙的人，即"钥匙保管员"。