ETA: This isn't going to be kind, but it's honest, and I hope some of you heed what I'm saying--
It's stuff like this that made me decide to stop teaching preschool. I was very good at what I did. But when parents are FURIOUS at the teacher for having *reasonable expectations* of children and then make a big stink over something their kid could just take away as a lesson.... ? I don't want to be part of the world where I have to pander and cater to such damn nonsense. (Just to interject, in case someone thinks I didn't let preschoolers go potty when they wanted... obviously, I did. Because that is *age-appropriate* for 3-5 year olds. Moving on...)
I'd get parents who were upset over the smallest things. Parents who ignored our preschool guidelines because they couldn't say 'no' to their kids, and then got upset because *I* said no. Or relied on me to be the one to say 'no' because they had to be the 'good guy' or were afraid their kid might get upset. We must be made of sterner stuff than that if we are to survive parenting our children AND teach them an appropriate amount of resilience.
Your child being upset over a mistake they made should not be a showstopper for you. It shouldn't result in your being furious and bugging the principal. I only emailed the principal when there was a serious safety concern. Otherwise, I was willing to work with and trust the teacher. I was willing for my son to be embarrassed, disappointed, etc if he made mistakes, because I knew that if he experienced those feelings, if I didn't come in and make him think he was being 'wronged', then he would LEARN from the experience.
In K and first grade, he didn't like the bathrooms, He didn't like the kids in there and he thought it would be 'embarrassing' to use them. In first grade, he wet his pants and then he discovered wetting himself was much more embarrassing than just using the bathroom. I brought him a change of clothes and then, even though he wanted to go home, I sent him back into the classroom for the last 40 minutes of instruction. "Honey, everyone in there has had an accident. Don't say anything about it and no one will either." Sure enough, I was right.
If you want to keep good teachers, this is how we start. By taking a minute when we are angry and then, when we are in control of ourselves, asking a few questions before jumping to conclusions that they are trying to hurt your kid, are out to get them, or assuming your kid can't handle it. Treat them as you would want to be treated. If you were at work and made a mistake and someone had a misunderstanding about it, would you want them to go off at your supervisor about you OR would you appreciate the thoughtful approach on their part.. "I was unsure what you meant by....could you explain that again, so I'm clear on this?" Taking a moment to think rationally ('of course the teacher doesn't want kids having accidents in class! I wonder if there was a miscommunication between she and my son') goes a long way in making the world a bit of a better place for some teachers... and the principals who get distracted from much weightier tasks than a parent who is mad because their son didn't manage to go when they were meant to.
This sort of parenting is driving us, the ones who would help you by setting good limits, out of the business. And creates children who may not be able to manage life's much larger embarrassments and disappointments because "mommy" isn't handling them very well either.
Original post:
It was unfortunate that this happened, and I'm sure if you are thoughtful in your approach, this likely won't happen again.
It may be that there were other kids asking to use the bathroom. It may be that they only allow the kids to go one at a time. It may be that she was trying to give all of the students information. I'm not making excuses, but it may be that she really didn't know that it was *urgent* urgent and thought that he might have been able to make it until the bell rang.
Or maybe there was chaos in the classroom with a bunch of kids asking to do things and she just said no to everyone. I have seen moments like this. Don't tell me you've never said 'no' to your kids at a moment when the request just didn't 'click' because you had other things on your mind. I know I have, many many times.
Of course, you wanted to be called, but if it were at the end of the day, there may not have been enough time for her to discreetly get to a place to call you. The classroom phone conversations are not private. (I've volunteered in the school and have witnessed this.) Calling you may have resulted in actually drawing the kids' attention to the fact that he'd had an accident. Handling it with discretion in the moment was probably the best she could do.
My guess is that if she 'looked scared and tried to apologize' that she is aware of her mistake. If this were me, this isn't something I would be emailing admin on. It sounds like she's now very aware that she made a mistake and my guess is that it won't happen again. I'd just make a note of it mentally, write it down somewhere if you need to. It's early in the school year and everyone is finding their rhythm. She may end up being a great teacher or may end up having strong rules which are not reasonable for the age group. What I'm not hearing from you is that *she* directly and deliberately humiliated him or let him be humiliated by others. So, that's something to consider. I know you are angry, but my guess is that she wasn't trying to be *mean*. (Those teachers do exist, by the way. But I'm not reading that in your post.)