I created a classroom bathroom pass as a computer lab teacher and it's very handy. They can either leave it at their computer so you know they're out of the room, or you can have them take the bathroom pass with them so you know where they're at. This would work for classroom teachers too.
Fluttering Through First Grade: Bathroom Trip Tallies. They put a tally in the box of their student, classroom An easy way for you to see who is leaving the classroom too often to use the bathroom. Great to show to parents too at conferences.
"Go passes." Each student has two trips to the bathroom or water per day. Once both of their clips are gone they need to "buy" a pass with the money from their classroom jobs. Easy way to monitor how often they go and keep them in check.