I have put these up on the back of my classroom door and they have been very successful. With everything that goes on in the classroom, it can be easy to forget which child has gone to the toilet or how long for. I regularly check the back of the door throughout the day. It is much easier to know if a child has been out of the classroom for a longer than expected amount of time. We also have a rule that if there is already one girl in the toilet, another girl has to wait. The same rule applies for boys. For younger students, you may want to send them in pairs.