I never deny the children the right to go to the bathroom. You gotta go, you gotta go. That said, our school policy (I'm in a senior high school) the bathrooms are locked 5 minutes before the bell rings until 5 minutes after the next class starts. My classroom is right next door to the bathroom, so it's not a big disruption for me. All I ask of my kids is that they get up, take the boys/girls lav pass with my name on it and go, don't make tons of noise to disrupt. The way I figure it, they're in High School, almost college. They're capable of knowing when they need to use the lav and how not to disrupt everyone. A simple "excuse me please" works fine. If it is the same kid, at the same time (say the class is during a lunch hour) I'll get a bit suspicious, but I'll always talk it over with the kid, then a phone call home, then if need be an administrator.
I want my kids to ask questions - that's how they learn. Sometimes I'll deliberately make a mistake and see if they catch it. Then I say something to the effect of, "oops, my mistake - good listening!" or "see, everyone makes mistakes."