They do all the things that secretaries in most businesses do. But they also do a lot of jobs that nobody else in the school does. They are the key people to whom everyone comes for information. Students ask them everything. Parents phone constantly.
Teachers ask about all kinds of rules and records.
They type correspondence and tests. They make sure attendance records balance. They help find lost books and soothe over little hurts. They register all visitors and new students. They occasionally find lunch money when someone loses it.