When the kids want someone, they come to the door of the office and yell "Sensei!" which is completely ignored by the 50 people in the room who are all "Sensei."
Or some witty teacher will call back, "We're all Sensei - which one of us do you want?"
I am sometimes "sensei" and sometimes just "-san", and sometimes yobisute
(without honourific), depending on who I'm working with.
I've also been a "sensei" at a Kenshinkai demonstration for reasons I can't quite fathom.