Add a confetti animation when you get back from the bathroom in time! When you are late, make an angry noise.
The ability to turn this on and off in the admin setting would be good. For example, we would use the confetti, but not the angry noise
Especially if on a kiosk, I think this would be distracting and drawn unnecessary attention to the student. It should be enough to know that expired passes will result in a consequence.
Not sure about the angry noise, some kids are attention seekers and would come back late just for it to make that noise.