All during my college and grad school years, I can't remember a single instructor leaving the room to go to the restroom.
I can't remember ever doing it myself. (Not as a student nor as an instructor at whatever level.)
That's not to say it couldn't happen, or that there's anything wrong if it does, because sometimes you gotta go.
But it seems that we instructors plan ahead when we can.
So why is it that so many students seem to need to go? I mean, I probably average 1-2 students a week who leave class to use the restroom. Is it just the odds? Or the occasional poor planning?