When the green sign is posted the students (one at a time) may move their number next to the restroom sign and take the pass and go. If the red sign is posted, then they may not leave the classroom to take a break. (Unless of course it is an emergency.
"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.