Am I the a**hole for not letting my student use the bathroom during the first 20 minutes of class?

I teach 3rd grade. I've been having a problem with my students getting up and walking to the bathrooms. I ask them to use the bathroom right before class starts, and there's a bathroom right next to the room. Plus, I have a composting toilet in the room. So they have three options. <br><br>So if they ask me to go to the bathroom during class, I say "Yeah I'm sure you can wait." I'm absolutely not making them wait, and if they have an emergency, they can go. But I also I don't want to disrupt the class and give an excuse to leave. <br><br>Well, today, my student didn't have the chance to go to the bathroom before class started, and I didn't let her go during class. I always make sure my students know I don't like bathroom breaks during class, and I've never let someone go to the bathroom for more than 20 minutes during class. But I saw her trying to hold it in, and I could see her face was red.<br><br>The principal walked in on our lesson and overheard a conversation where I denied a student using the bathroom. She took me aside and said that I was basically depriving the student of her right to use the bathroom, I was depriving her of her dignity, and she said that it was wildly inappropriate to see the look on my students' face. I said it was only 20 minutes until lunch, but she said that it was the principal of the thing, that everyone knows that I don't let my students use the bathroom, and that it's disgusting.<br><br>I know it's the principal of the thing, but I really believe that as a teacher, it's my right to not let students go to the bathroom for a small amount of time. Plus, I know the students abuse that privilege. I don't know if I overstepped my boundaries but I don't think it's a big deal.