Momoyo calls him "Old Man", Wanko calls him "Gramps", the Teachers and students call him "Principal" .
Due to the magic of the script, nobody calls out to him, even when talking directly to him. Lu, for example, he never says his title or his name, even though he's the one who talks to him the most.
If you continue having such reactions I can guarantee that they won't stop. Ignore them, lad. Ignore them. ("them" implies I'm not in on it)