If you went through traditional schooling, you may be able to recall feelings of anxiety about having to go to the bathroom, especially if you were an introvert. In what other context must you (effectively) announce to everyone in the room that you are now about to go pee or poop? That your bladder couldn't make it until recess? Random thought: even prisoners don't have to do that.
In first or second grade, I remember the huge puddle that one of my classmates left on the floor under his desk. If it was an indelible memory for me, think what it must have been like for him! I remember someone had to come in and mop it all up, and he had to get new clothes. What on earth would cause someone to hold it to the breaking point? There's a bathroom right down the hall! My bet is that it was because he couldn't muster the courage to ask, or possibly couldn't handle the perceived shame of asking, permission to pee.
What's so strange about this? What am I getting at? It may seem natural to you if you went to traditional school. You sit in your seat and do as you're told. You can only get up with permission from the teacher. Of course, this is not natural, especially for rambunctious little kids. But even if you ignore that for a moment, surely you must realize, with a little reflection, that it's not dignified for anyone to have to ask to pee. What if we're robbing our kids of their dignity? What if this is just one (particularly pointed) example of how we do it?
My wife told me she used to imagine that her crayons were popsicles, because she was so thirsty while stuck at her desk, waiting for school to get out. She would avoid drinking earlier so she wouldn't have to pee, and then she'd get overly thirsty. I wonder what she really learned all those afternoons at school...
So if you went to school, reflect back on your experiences. Did you have any of these bladder- or thirst-related moments of anxiety? Are they just a necessary part of growing up? Are you willing to put your kids through it too, now that you've thought about it?