For my context here, I went to a high school that was centered around progressive education principles and I never had to ask to go to the bathroom. There were no disciplinary policies like that. And in fact, everything was fine. We all still worked hard, paid attention, and got our diplomas. I think too many teachers assume students are all malicious gremlins trying as hard as they can to avoid school. Turns out, most adults will start to chafe too if you tell them they have to ask to go to the bathroom after the third time doing so in your presence, let alone if you make them ask every time. And some adults will even use bathroom breaks as an excuse to waste time.

So be it! That's life.

I'm not going to assume anything because it may have just been what you told the children, but if you actually sent notes to the nurse without talking to a kid beforehand, you're definitely overstepping.

It's perfectly normal for bowel movements to be on a schedule. If a student it leaving your class multiple time per week then have a discussion to see what's going on, you might learn something about their life that paints them in a new light. Or, maybe discuss the possibility of going before class starts if possible.

I think people are downvoting your original comment because referring a student directly to the nurse is a strange thing to threaten upfront.